GIFT   OF 
MICHAEL  REESE 


ORGANIC   CHEMISTRY 


PART    II. 


BY 


W.  H.  PERKIN,  JUN.,  Ph.D.,  F.R.S. 

PROFESSOR  OF  ORGANIC   CHEMISTRY   IN  OWENS   COLLEGE,    MANCHESTER 

AND 

F.  STANLEY  KIPPING,  Ph.D.,  D.Sc.  (LoxD.),  F.R.S. 

PROFESSOR   OF  CHEMISTRY   IN   UNIVERSITY   COLLEGE,    NOTTINOHAM 


EDINBURGH  AND  LONDON 

W.  &   R.   CHAMBERS,   LIMITED 

PHILADELPHIA:   J.  B.  LIPPINCOTT  COMPANY 


PEEFACE. 


THE  present  volume  (Part  II.)  consists  principally  of  a 
description  of  the  aromatic  compounds,  and,  together  with 
Part  L,  forms  an  introduction  to  Organic  Chemistry. 

The  opening  chapters  of  Part  II.  contain  an  account  of  coal- 
tar  and  its  treatment.  This  leads  naturally  to  a  description  of 
the  preparation  and  properties  of  benzene,  and  to  a  discussion 
of  its  constitution  in  the  light  of  facts  previously  dealt  with ; 
the  student  is  thus  made  acquainted  with  the  principal 
characteristics  of  aromatic,  as  distinct  from  fatty,  compounds, 
and  is  then  in  a  position  to  understand  the  classification  of 
organic  substances  into  these  two  main  divisions. 

The  more  important  classes  of  aromatic  compounds  are  then 
described,  but  in  a  somewhat  different  manner  from  that 
adopted  in  Part  L,  inasmuch  as  a  general  account  of  the 
properties  of  each  class  of  substances  is  given  before,  instead 
of  after,  the  more  detailed  description  of  typical  compounds ; 
this  course  is  to  a  great  extent  free  from  the  disadvantages 
which  are  found  to  attend  its  adoption  at  earlier  stages,  as  the 
student  has  by  this  time  acquired  some  experience  of  the  more 
systematic  method  from  a  study  of  the  summaries  given  in 
Part  I. 

Special  attention  has  been  given,  as  before,  to  questions  of 
constitution,  one  of  the  objects  being  to  train  the  student  to 
think  out  such  matters,  and  to  try  and  deduce  a  constitutional 

96721 


IV  PREFACE. 

formula  for  a  given  substance,  by  comparing  its  properties  with 
those  of  others  of  known  constitution  ;  with  this  end  in  view, 
it  has  often  been  thought  desirable  to  withhold  the  most 
important  evidence  in  favour  of  the  accepted  constitutional 
formula  until  the  subject  had  been  discussed  at  some  length. 

The  concluding  chapters  on  dyes,  alkaloids,  and  stereo- 
isomerism  will  doubtless  offer  the  greatest  difficulties,  but, 
considering  the  importance  of  the  matters  with  which  they 
deal,  their  omission  or  curtailment  was  deemed  inadvisable. 
The  account  of  the  alkaloids  should  be  useful,  more  particu- 
larly to  medical  students,  whilst  the  chapter  on  dyes  deals 
with  a  variety  of  substances  of  even  greater  practical  value, 
and  indicates  the  methods  employed  in  one  of  the  most  im- 
portant applications  of  organic  chemistry.  The  chapter  on 
stereo-isomerism  was  included  because,  owing  to  the  import- 
ance to  which  this  theory  has  now  attained,  a  text-book  on 
organic  chemistry  would  be  incomplete  without  a  brief  dis- 
cussion of  the  subject.  The  full  directions  which  are  given 
for  the  use  of  models  will,  it  is  hoped,  lead  to  a  clear  con- 
ception of  the  views  set  forth. 

The  practical  aspect  of  the  science  has  again  been  kept  well 
to  the  front,  a  detailed  description  of  the  preparation  of  all 
the  more  typical  compounds  being  given  (usually  in  smaller 
type),  in  order  to  facilitate  the  laboratory  work,  which  must  be 
regarded  as  a  necessary  accompaniment  to  the  theoretical 
knowledge. 

Our  thanks  are  again  due  to  Dr  A.  Harden  for  many 
valuable  suggestions,  as  well  as  for  help  in  revising  the 
proof-sheets,  and  in  preparing  the  index. 


CONTENTS. 


PAGE 

CHAPTER  XVII. — MANUFACTURE,  PURIFICATION,  PROPERTIES, 

AND  CONSTITUTION  OF  BENZENE 295 

CHAPTER  XVIIL— ISOMERISM  OF  BENZENE  DERIVATIVES,  AND 

DETERMINATION  OF  THEIR  CONSTITUTION 310 

CHAPTER  XIX.— GENERAL   PROPERTIES  OF  AROMATIC    COM- 
POUNDS  322 

Classification  of  Organic  Compounds 322 

General  Character  of  Aromatic  Compounds 324 

CHAPTER  XX.  — HOMOLOGUES  OF  BENZENE 328 

Toluene— Xylenes — Mesitylene— Cumene — Gymene 334-339 

Diphenyl — Diphenylmethane— Triphenylmethane 340 

CHAPTER  XXI.— HALOGEN  DERIVATIVES  OF  BENZENE  AND  ITS 

HOMOLOGUES 341 

Chlorobenzene  —  Bromobenzene  —  Chlorotoluene  —  Benzyl 
Chloride 347,  348 

CHAPTER  XXII.— NITRO-COMPOUNDS 350 

Nitrobenzene — Meta-dinitrobenzene — Nitrotoluenes 352-355 

CHAPTER  XXIII. — AMIDO-COMPOUNDS  AND  AMINES 355 

Aniline  and  its  Derivatives 361 

Homologues  of  Aniline — Alky lanilines 364 

Dipheny lamine  and  Tripheny lamine 367 

Aromatic  Amines — Benzylamine 368 

CHAPTER  XXIV. — DIAZO-COMPOUNDS  AND  DERIVATIVES 370 

Diazoamido-  and  Amidoazo-compounds 374 

Phenylhydrazine 376 

Azo-compounds 377 

CHAPTER  XXV.— SULPHONIC  ACIDS  AND  THEIR  DERIVATIVES... 379 

CHAPTER  XX VI. —PHENOLS 385 

Monohydric  Phenols— Phenol— Picric  Acid— Cresols  ....391-396 
Dihydric  Phenols— Catechol,  Resorcinol,  Hydroquinone..398, 399 
Trihydric  Phenols 399 

CHAPTER     XXVII.  —  AROMATIC     ALCOHOLS,     ALDEHYDES, 

KETONES,  AND  QUINONES 402 

Alcohols— Benzyl  Alcohol 402,  403 

Aldehydes— Benzaldehyde 405 

Hyd roxy-aldehydes— Salicylaldehyde 408,  409 

-Ketones — Acetophenone 411 

Quinones — Quinone 413 


VI  CONTENTS. 

PAGE 

CHAPTER  XXVIII.— CARBOXYLIC  ACIDS 416 

Berizoic    Acid — Benzoyl    Chloride — Benzoic    Anhydride  — 

Benzamide — Berizonitrile 418-421 

Substitution  Products  of  Benzoic  Acid 422 

Toluic  Acids 423 

Dibasic  Acids — Phthalic   Acid,   Phthalic   Anhydride,  Iso- 

phthalic  Acid,  Terephthalic  Acid 423-427 

Phenylacetic  Acid,  Phenylpropionic  Acid,  and  Derivatives.. 427 

Cinnamic  Acid 430 

CHAPTER  XXIX.  — HYDROXYCARBOXYLIC  ACIDS 433 

Salicylic  Acid — Anisic  Acid— Protocatechuic  Acid — Gallic 

Acid — Tannin— Mandelic  Acid 437-440 

CHAPTER  XXX. — NAPHTHALENE  AND  ITS  DERIVATIVES 442 

Naphthalene 442 

Naphthalene    Tetrachloride  —  Nitro-derivatives  —  Amido- 
derivatives  —  Naphthols  —  Sulphonic   Acids  —  a-Naph- 

thaquinone — /3-Naphthaquinone 450-456 

CHAPTER  XXXI. —ANTHRACENE  AND  PHENANTHRENE, 457 

Anthracene 457 

Anthraquinone  —  Alizarin  —  Phenanthrene  —  Phenanthra- 

quinorie — Diphenic  Acid 462-471 

CHAPTER  XXXIL — PYRIDINE  AND  QUINOLINE 471 

Pyridine  and  its  Derivatives 472 

Piperidine 476 

Homologues  of  Pyridine — Pyridinecarboxylic  Acids 478 

Quinoline 480 

Secondary  and  Tertiary  Aromatic  Bases 483 

CHAPTER  XXXIII. — ALKALOIDS 484 

Alkaloids  derived  from  Pyridine,  488  ;  from  Quinoline 492 

Alkaloids  contained  in  Opium — Morphine,  &c 495 

Alkaloids  related  to  Uric  Acid— Caffeine,  &c 497 

Antipyrine,  Kairine,  Thalline 499 

Choline,  Beta'ine,  Neurine,  and  Taurine 500 

CHAPTER  XXXIV.  —DYES  AND  THEIR  APPLICATION 502 

Malachite  Green,  Pararosaniline,  Rosaniline,  Methylviolet, 

Aniline  Blue 509-517 

The  Phthaleins— Phenolphthalein,  Fluorescei'n,  Eosin.. 518-521 
Azo-dyes — Aniline  Yellow,  Chrysoidine,  Bismarck  Brown, 
Helianthin,    Ptesorcin    Yellow,    Rocellin,     Congo -red, 

Benzopurpurins 522-526 

Various  Colouring  Matters — Martins'  Yellow,  Methylene 

Blue,  Indigo 527 

CHAPTER  XXXV.— STEREO-ISOMERISM 528 


ORGANIC     CHEMISTRY. 


PAET   II. 


CHAPTER    XVII. 

.MANUFACTURE,     PURIFICATION,    PROPERTIES,    AND    CONSTITUTION 
OF   BENZENE. 

Distillation  of  Coal-tar.— When  coal  is  strongly  heated  out 
of  contact  with  air,  it  undergoes  very  complex  changes,  and 
yields  a  great  variety  of  gaseous  and  liquid  products,  together 
with  a  solid,  non-volatile  residue  of  coke.  This  process  of 
dry  or  destructive  distillation  is  carried  out  on  the  large  scale 
in  the  manufacture  of  coal-gas,  1'or  which  purpose  the  coal  is 
heated  in  clay  or  iron  retorts,  provided  with  air-tight  doors ; 
the  gas  and  other  volatile  products  escape  from  the  retort 
through  a  pipe,  and  when  distillation  is  at  an  end,  the  coke,  a 
porous  mass  of  carbon,  containing  the  ash  or  mineral  matter 
of  the  coal,  is  withdrawn. 

The  hot  coal-gas  passes  first  through  a  series  of  pipes  or 
conrfwwr*,  kept  cool  by  immersion  in  watej  or  simply  by 
exposure  to  the  air,  and,  as  its  temperature  falls,  it  deposits  a 
considerable  quantity  of  tar  and  gas-liquor,  which  are  run 
together  into  a  large  tank ;  it  is  then  forced  through,  or 
washed  with,  water,  in  washers  and  scrubbers,  and,  after  having 
been  further  freed  from  tar,  ammonia,  carbon  dioxide,  and 
sulphuretted  hydrogen  by  suitable  processes  of  purilication,  it 


296  -MANUFACTURE,    PURIFICATION,    PROPERTIES, 

is  led  into  the  gas-holder  and  used  for  illuminating  and  heating 
purposes.  The  average  volume  percentage  composition  of  puri- 
fied coal-gas  is  H2  =  47,CH4  =  36,CO  =  8,C02  -  1,N2  =  4, 
and  hydrocarbons,  other  than  marsh-gas  (acetylene,  ethylene, 
benzene,  &c.)  =  4. 

The  coal-tar  and  the  gas-liquor  in  the  tank  separate  into 
two  layers ;  the  upper  one  consists  of  gas-liquor  or  ammoniacal- 
liquor  (a  yellow,  unpleasant-smelling,  aqueous  solution  of 
ammonium  carbonate,  ammonium  sulphide,  and  numerous  other 
compounds),  from  which  practically  the  whole  of  the  ammonia 
and  ammonium  salts  of  commerce  are  obtained.  The  lower 
layer  in  the  tank  is  a  dark,  thick,  oily  liquid  of  sp.  gr.  1-1  to 
1-2,  known  as  coal-tar.  It  is  a  mixture  of  a  great  number  of 
organic  compounds,  and,  although  not  long  ago  it  was  con- 
sidered to  be  an  obnoxious  bye-product,  it  is  now  the  sole 
source  of  very  many  substances  of  great  industrial  importance. 

In  order  to  partially  separate  the  several  constituents,  the 
tar  is  submitted  to  fractional  distillation ;  it  is  heated  in  large 
wrought-iron  stills  or  retorts,  and  the  vapours  which  pass  off 
are  condensed  in  long  iron  or  lead  worms  immersed  in  water, 
the  liquid  distillate  being  collected  in  fractions.  The  point  at 
which  the  receiver  is  changed  is  ascertained  by  means  of  a 
thermometer,  which  dips  into  the  tar,  as  well  as  by  the 
character  of  the  distillate. 

In  this  way  tar  may  be  roughly  separated  into  the  following 
fractions : 

I.  Light  oil  or  crude  naphtha Collected  up  to  170°. 

II.  Middle  oil  or  carbolic  oil „        between  170  and  230°. 

III.  Heavy  oil  or  creosote  oil „  „        230     „    270°. 

IV.  Anthracene  oil „        above  270°. 

V.  Pitch Residue  in  the  still. 

I.  The  first  crude  fraction  separates  into  two  layers — namely, 
gas-liquor  (which  the  tar  always  retains  mechanically  to  some 
extent)  and  an  oil  which  is  lighter  than  water,  its  sp.  gr. 
being  about  0-975,  hence  the  name,  light  oil.  This  oil  is 
first  redistilled  from  a  smaller  iron  retort  and  the  distillate 


AND    CONSTITUTION    OF    BENZENE.  297 

collected  in  three  principal  portions,  passing  over  between 
82-110°,  110-140°,  and  140-170°  respectively.  All  these 
fractions  consist  principally  of  hydrocarbons,  but  contain  basic 
substances,  such  as  pyridine,  acid  substances,  such  as  phenol 
or  carbolic  acid,  and  various  other  impurities ;  they  are, 
therefore,  separately  agitated,  first  with  concentrated  sulphuric 
acid,  which  dissolves  out  the  basic  substances,  and  then  with 
caustic  soda,  which  removes  the  phenols  (p.  385),  being  washed 
with  water  after  each  treatment ;  afterwards  they  are  again 
distilled.  The  oil  obtained  in  this  way  from  the  fraction 
collected  between  82  and  110°  consists  principally  of  the 
hydrocarbons  benzene  and  toluene,  and  is  sold  as  '  90  per 
cent,  benzol;'  that  obtained  from  the  fraction  110-140° 
consists  essentially  of  the  same  two  hydrocarbons  (but  in 
different  proportions)  together  with  xylene,  and  is  sold  as 
'  50  per  cent,  benzol.'*  These  two  products  are  not  usually 
further  treated  by  the  tar-distiller,  but  are  worked  up  in  the 
manner  described  later.  The  oil  from  the  fraction  collected 
between  140-170°  consists  of  xylene,  pseudocumene,  mesityl- 
ene,  &c.,  and  is  principally  employed  as  '  solvent  naphtha,' 
also  as  '  burning  naphtha.' 

II.  The  second  crude  fraction,  or  middle  oil,  collected 
between  170  and  230°,  has  a  sp.  gr.  of  about  1*002,  and  con- 
sists principally  of  naphthalene  and  carbolic  acid.  On  cooling, 
the  naphthalene  separates  in  crystals,  which  are  drained  and 
pressed  to  squeeze  out  adhering  carbolic  acid  and  other  sub- 
stances ;  the  crude  crystalline  product  is  further  purified  by 
treatment  with  caustic  soda  and  sulphuric  acid  successively, 
and  finally  sublimed  or  distilled.  The  oil  from  which  the 
crystals  have  been  separated  is  agitated  with  warm  caustic 
soda  to  dissolve  the  carbolic  acid ;  the  alkaline  solution  is 
then  drawn  off  from  the  insoluble  portions  of  the  oil  and 

*  Commercial  '90  per  cent,  benzol'  contains  about  70  per  cent.,  and  '50 
per  cent,  benzol '  about  46  per  cent,  of  pure  benzene ;  the  terms  refer  to 
the  proportion  of  the  mixture  which  passes  over  below  100°  when  the  com- 
mercial product  is  distilled.  Benzene,  toluene,  and  xylene  are  known  com- 
mercially as  benzol,  toluol,  and  xylol  respectively. 


298  MANUFACTURE,    PURIFICATION,    PROPERTIES, 

treated  with  sulphuric  acid,  whereupon  crude  carbolic  acid 
separates  as  an  oil,  which  is  washed  with  water  and  again 
distilled ;  it  is  thus  separated  into  crystalline  (pure)  carbolic 
acid  and  liquid  (impure)  carbolic  acid. 

III.  The  third  crude  fraction,  collected  between  230  and 
270°  is  a  greenish-yellow,  fluorescent  oil,  specifically  heavier 
than  water;    it  contains  carbolic  acid,  cresol,  naphthalene, 
anthracene,  and  other  substances,  and  is  chiefly   employed 
under   the   name  of   'creosote  oil'  for  the    preservation   of 
timber. 

IV.  The  fourth   crude  fraction,  collected  at  270°  and  up- 
wards,   consists    of    anthracene,    phenanthrene,    and    other 
hydrocarbons  which  are  solid  at  ordinary  temperatures ;   the 
crystals  which  are  deposited   on  cooling,  after  having  been 
freed  from   oil  by  pressure,   contain  about   30   per  cent,   of 
anthracene,  and  are  further  purified  by  digestion  with  solvent 
naphtha,  which  dissolves  the  other  hydrocarbons  more  readily 
than   the  anthracene ;    the  product  is  then  sold  as  '  50  per 
cent,   anthracene,'  and  is  employed  in   the  manufacture   of 
alizarin   dyes.      The  oil  drained  from  the  anthracene  is. re- 
distilled, to  obtain  a  further  quantity  of  the  crystalline  product, 
the  non-crystallisable  portions  being  known  as  'anthracene 
oil' 

V.  The  pitch  in  the  still  is  run  out  while  still  hot,  and  is 
employed  in  the  preparation  of  varnishes,  for  protecting  wood 
and  metal  work,  and  in  making  asphalt. 

The  following  table,  taken  partly  from  Ost's  Lelirlmch  der 
teclmisclien  Cheinie,  shows  in  a  condensed  form  the  process 
of  tar  distillation  and  the  more  important  commercial  products 
obtained. 

Benzene,  C6H6. — The  crude  '90  per  cent,  benzol'  of  the 
tar-distiller  consists  essentially  of  a  mixture  of  benzene  and 
toluene,  but  contains  small  quantities  of  xylene  and  other 
substances ;  on  further  fractional  distillation  in  specially  con- 
structed apparatus  (similar  to  that  employed  in  the  rectifica- 
tion of  spirit),  it  is  separated  more  or  less  completely  into  its 


AND    CONSTITUTION    OF   BENZENE. 


299 


300  MANUFACTURE,    PURIFICATION,    PROPERTIES, 

constituents.  The  benzene  prepared  in  this  way  still  contains 
small  quantities  of  toluene,  paraffins,  carbon  bisulphide,  and 
other  impurities,  and  rnay  be  further  treated  in  the  following 
manner :  It  is  first  cooled  in  a  freezing  mixture  and  the 
crystals  of  benzene  quickly  separated  by  filtration  from  the 
mother-liquor,  which  contains  most  of  the  impurities ;  after 
repeating  this  process,  the  benzene  is  carefully  distilled,  and 
the  portion  boiling  at  80-81°  collected  separately.  For 
ordinary  purposes  this  purification  is  sufficient,  but  even  now 
the  benzene  is  not  quite  pure,  and,  when  it  is  shaken  with 
cold  concentrated  sulphuric  acid,  the  latter  darkens  in  colour 
owing  to  its  having  charred  and  dissolved  the  impurities ; 
pure  benzene,  on  the  other  hand,  does  not  char  with  sulphuric 
acid,  so  that  if  the  impure  liquid  be  repeatedly  shaken  with 
small  quantities  of  the  acid,  until  the  latter  ceases  to  be  dis- 
coloured, most  of  the  foreign  substances  will  be  removed. 

All  coal-tar  benzene,  which  has  not  been  purified  by  repeated 
treatment  with  sulphuric  acid,  contains  an  interesting  sulphur 
compound,  C4H4S,  named  thiophene,  which  was  discovered  by  V. 
Meyer ;  the  presence  of  this  substance  is  readily  detected  by  shak- 
ing the  sample  with  a  little  concentrated  sulphuric  acid  and  a  trace 
of  isatin  (an  oxidation  product  of  indigo),  when  the  acid  assumes  a 
beautiful  blue  colour  (indophenin  reaction) ;  thiophene  resembles 
benzene  very  closely  in  chemical  and  physical  properties,  and  for 
this  reason  cannot  be  separated  from  it  except  by  repeated  treat- 
ment with  sulphuric  acid,  which  dissolves  thiophene  more  readily 
than  it  does  the  hydrocarbon. 

Although  the  whole  of  the  benzene  of  commerce  ('  benzol ') 
is  prepared  from  coal-tar,  the  hydrocarbon  is  also  present  in 
small  quantities  in  wood-tar  and  in  the  tarry  distillate  of  many 
other  substances,  such  as  shale,  peat,  &c. ;  it  may,  in  fact,  be 
produced  by  passing  the  vapour  of  alcohol,  ether,  petroleum, 
or  of  many  other  volatile  organic  substances  through  a  red-hot 
tube,  because  under  these  conditions  such  compounds  lose 
hydrogen  (and  oxygen),  and  are  converted  into  benzene  and 
its  derivatives. 

Benzene  may  be   produced   synthetically  by  simply  heating 


AND    CONSTITUTION    OF    BEXZEXE. 


301 


ncotyleno  at  a  dull-red   heat,  when  3  mols.  (or  6  vols.)  of  the 
latter  are  converted  into  1  mol.  (or  2  vols.)  of  benzene, 


Acetylene,  generated  from  its  copper  derivative  (part  i.  p.  83),  is 
collected  over  mercury  in 
a  piece  of  hard  glass- 
tubing,  closed  at  one  end 
and  bent  at  an  angle  of 
about  120°;  when  the  tube 
is  about  half  full  of  gas, 
the  lower  end  is  closed 
with  a  cork,  and  a  piece 
of  copper  gauze  wrapped 
round  a  portion  of  the 
horizontal  limb,  as  shown 
(fig.  19).  This  portion  of 
the  tube  is  then  carefully 
and  strongly  heated  with 
a  bunsen  burner,  the  other 
end  remaining  immersed 
in  the  mercury;  after  a 
short  time  vapours  appear 
in  the  tube,  and  minute 
drops  of  benzene  condense 
on  the  sides,  and  if,  after 
heating  for  about  fifteen 
minutes,  the  tube  be 

allowed  to  cool  and  the  cork  then  removed,  the  mercury  will  rise, 
showing  that  a  diminution  in  volume  has  taken  place. 

This  conversion  of  acetylene  into  benzene  is  a  process  of 
polymerisation,  and  was  first  accomplished  by  Berthelot.  It 
is,  at  the  same  time,  an  exceedingly  important  synthesis  of 
benzene  from  its  elements,  because  acetylene  may  be  obtained 
by  the  direct  combination  of  carbon  and  hydrogen. 

Pure  benzene  may  be  conveniently  prepared  in  small 
quantities  by  heating  pure  benzoic  acid  or  calcium  benzoate 
with  soda-lime,  a  reaction  which  recalls  the  formation  of 
marsh-gas  from  calcium  acetate, 

(C6H5.COO)2Ca  +  2NaOH  =  2C6H6  +  CaC03  +  Na0COg, 
or  C6H5<COOH  =  C6H6  +  C03. 


Fig.  19. 


302  MANUFACTURE,    PURIFICATION,    PROPERTIES, 

At  ordinary  temperatures  benzene  is  a  colourless,  highly- 
refractive,  mobile  liquid  of  sp.  gr.  0-8799  at  20°,  but  when 
cooled  in  a  freezing  mixture  it  solidifies  to  a  crystalline  mass, 
melting  again  at  6°,  and  boiling  at  80-5°.  It  has  a  burning 
taste,  a  peculiar,  not  unpleasant  smell,  and  is  highly  inflam- 
mable, burning  with  a  luminous,  very  smoky  flame,  which  is 
indicative  of  its  richness  in  carbon ;  the  luminosity  of  an 
ordinary  coal-gas  flame  is,  in  fact,  largely  due  to  the  presence 
of  benzene.  Although  practically  insoluble  in  water,  benzene 
mixes  with  liquids  such  as  alcohol,  ether,  and  petroleum  in 
all  proportions  ;  like  the  latter,  it  readily  dissolves  fats,  resins, 
iodine,  and  other  substances  which  are  insoluble  in  water, 
and  is  for  this  reason  extensively  used  as  a  solvent  and  for 
cleaning  purposes;  its  principal  use,  however,  is  for  the 
manufacture  of  nitrobenzene  (p.  352)  and  other  benzene 
derivatives. 

Benzene  is  a  very  stable  substance,  and  is  resolved  into 
simpler  substances  only  with  great  difficulty ;  when  boiled 
with  concentrated  alkalies,  for  example,  it  undergoes  no 
change,  and  even  when  heated  with  solutions  of  such  power- 
ful oxidising  agents  as  chromic  acid  or  potassium  permangan- 
ate, it  is  only  very  slowly  attacked  and  decomposed,  carbon 
dioxide  and  traces  of  other  substances  being  formed.  Under 
certain  conditions,  however,  benzene  readily  yields  substitution 
products  ;  concentrated  nitric  acid,  even  at  ordinary  tempera- 
tures, converts  the  hydrocarbon  into  nitrobenzene  by  the  sub- 
stitution of  the  monovalent  nitro-group  -NO^,  for  an  atom  of 
hydrogen, 

C6H6  +  HN03  -  C6H5.N02  +  H20, 

and  concentrated  sulphuric  acid,  slowly  at  ordinary  tempera- 
tures, but  more  rapidly  on  heating,  transforms  it  into  benzene- 
sulphonic  acid, 

C6H6  +  H2S04  =  C6H5-S03H  +  H20. 

The  action  of  chlorine  and  bromine  on  benzene  is  very 
remarkable  :  at  moderately  high  temperatures,  or  in  presence 


AND    CONSTITUTION    OF    BENZENE.  303 

of  direct  sunlight,  it  is  rapidly  converted  into  additive  pro- 
ducts, such  as  benzene  hexachloride,  C6H6C16,  and  benzene 
hexabromide,  C6H6Br6,  by  direct  combination  with  six  (but 
never  more  than  six)  atoms  of  the  halogen;  in  absence  of 
sunlight  and  at  ordinary  temperatures,  however,  the  hydro- 
carbon is  slowly  attacked,  yielding  substitution  products,  such 
as  chlorobenzene,  C6H5C1,  bromobenzene,  C6H5Br,  dichloro- 
benzene,  C6H4C12,  &c. ;  when,  again,  some  halogen  carrier 
(p.  342),  such  as  ferric  chloride,  iodine,  &c.,  is  present,  action 
takes  place  readily  at  ordinary  temperatures  even  in  the  dark, 
and  substitution  products  are  formed. 

Constitution  of  Benzene. — It  will  be  seen  from  these  facts 
that  although  benzene,  like  the  paraffins,  is  an  extremely  stable 
substance,  it  differs  from  them  very  considerably  in  chemical 
behaviour,  more  especially  in  being  comparatively  readily 
acted  on  by  nitric  acid,  sulphuric  acid,  and  halogens,  and  in 
forming  additive  products  with  the  last  named  under  certain 
conditions ;  if,  again,  its  properties  be  compared  with  those 
of  the  unsaturated  hydrocarbons  of  the  ethylene  or  acetylene 
series,  the  contrast  is  even  more  striking,  particularly  when 
it  is  borne  in  mind  that  the  molecular  formula  of  benzene/ 
C6H6,  indicates  a  relation  to  these  unsaturated  hydrocarbons 
rather  than  to  the  saturated  compounds  of  the  methane 
series.  » 

In  order,  then,  to  obtain  some  clue  to  the  constitution  of 
benzene,  it  is  clearly  of  importance  to  carefully  consider 
the  properties  of  other  unsaturated  hydrocarbons  of  known 
constitution,  and  to  ascertain  in  what  respects  they  differ 
from  benzene;  for  this  purpose  the  compound  dipropargyl 
may  be  chosen,  as  it  has  the  same  molecular  formula  as 
benzene. 

Dipropargyl,  C6H6,  is  obtained  as  follows  :  diallyl  is  first  pre- 
pared by  treating  allyl  iodide  with  sodium, 

2CH2:CH-CH2I  +  2Na  =  CH2:CH.CH2.CH2.CH:CH2  +  2NaI; 
diallyl    combines    directly  with    bromine,  yielding  diallyl  tetra,-* 


304  MANUFACTURE,    PURIFICATION,    PROPERTIES, 

bromide,  and  this,  on   treatment  with   alcoholic  potash,   loses  4 
molecules   of    hydrogen    bromide,    and  is    converted    into    dipro- 
pargyl, 
CH2Br.CHBr.CH2-CH2.CHBr.CH2Br  = 

CH I  C.CH2.CH2-C  !  CH  +  4HBr. 

Now  although  dipropargyl  and  benzene  are  isomeric  and 
similar  in  ordinary  physical  properties,  they  are  absolutely 
different  in  chemical  behaviour ;  the  former  is  very  unstable, 
readily  undergoes  polymerisation,  combines  energetically  with 
bromine,  giving  additive  compounds,  and  is  immediately 
oxidised  even  by  weak  agents ;  it  shows,  in  fact,  all  the 
properties  of  an  unsaturated  hydrocarbon  of  the  acetylene 
series.  Benzene,  on  the  other  hand,  is  extremely  stable,  is 
comparatively  slowly  acted  on  by  bromine,  giving  (usually) 
substitution  products,  and  is  oxidised  only  very  slowly  even 
by  the  most  powerful  agents.  Since,  therefore,  dipropargyl 
must  be  represented  by  the  above  formula  in  order  to 
account  for  its  method  of  formation  and  chemical  properties, 
the  constitution  of  benzene  could  not  possibly  be  expressed 
by  any  similar  formula,  such  as 

CH3.C;C-C;C-CH3  or  CH2:C:CH.CH:C:CH2, 
because  compounds  similar  in  constitution  are  always  more  or 
less  similar  in  properties,  and  such  a  formula,  therefore,  would 
not  afford  the  slightest  indication  of  the  enormous  differences 
between  benzene  and  dipropargyl. 

This,  and  many  other  reasons  which  will  be  stated  later,  led 
to  the  conclusion  that  the  six  carbon  atoms  in  benzene  form 
a  closed-chain  or  nucleus  as  represented  by  the  symbol 


or 


c 

and  this  view,  first  suggested  by  Kekule*  in  1865,  is  now 
universally  accepted  as  the  best  explanation  of  the  behaviour 
of  benzene.  Kekule  also  pointed  out  that  numerous  facts 


AND    CONSTITUTION    OF    BENZENE.  305 

established  during  the  study  of  the  derivatives  of  benzene, 
admit  of  only  one  conclusion — namely,  that  the  molecule  of 
benzene  is  symmetrical,  and  that  each  carbon  atom  is  directly 
united  with  one  (and  only  one)  atom  of  hydrogen,  as  repre- 
sented by  the  formula 
H 

A 


or 


Of  these,  however,  the  former  is  always  used  in  preference  to 
the  latter,  partly  because  straight  lines  are  invariably  em* 
ployed  to  represent  direct  union  between  two  atoms,  and 
partly  on  account  of  certain  views  which  are  discussed  in  a 
later  chapter  (p.  528). 

Up  to  this  point  all  chemists  are  agreed,  as  the  evidence 
which  can  be  brought  forward  in  support  of  this  formula  is 
simply  overwhelming;  nevertheless,  at  least  one  important 
matter  has  still  to  be  settled,  before  it  can  be  said  that  the 
constitution  of  benzene  is  established  as  far  as  present  theories 
permit.  The  point  referred  to  is,  the  manner  in  which  the 
carbon  atoms  are  united  with  one  another.  The  whole  theory 
of  the  constitution  of  organic  compounds  is  based  on  the 
assumption  that  carbon  is  always  tetravalent,  and  this 
assumption,  as  already  explained  (part  i.  p.  53),  is  expressed 
in  graphic  formula  by  drawing  four  lines  from  each  carbon 
atom,  in  such  a  way  as  to  show  in  what  manner,  and  to 
which  other  atoms,  the  particular  carbon  atom  in  question  is 
directly  united.  Xow,  if  this  be  done  in  the  case  of  benzene, 
it  is  clear  that  two  of  the  four  lines  or  bonds,  which  represent 
the  valency  of  each  carbon  atom,  must  be  drawn  to  meet  two 
other  carbon  atoms,  because  unless  each  carbon  atom  is 
directly  united  with  two  others,  the  six  could  not  together 
form  a  closed-chain  j  a  third  line  or  bond  is  easily  accounted 

T 


306 


MANUFACTURE,    PURIFICATION,    PROPERTIES, 


for,  because  each  carbon  atom  is  directly  united  with  hydrogen. 
In  this  way,  however,  only  three  of  the  four  affinities  of  each 
carbon  atom  are  disposed  of,  whereas  it  is  assumed  that  carbon 
is  always  tetravalent,  and  it  is  known  that  each  of  the  carbon 
atoms  in  benzene  is  still  capable  of  combining  with  one  mono- 
valent  group  or  atom. 

The  next  question,  then,  to  be  considered  is,  how  may  the 
fourth  affinity  or  combining  power  of  each  carbon  atom  be  re- 
presented so  as  to  give  the  clearest  indication  of  the  behaviour 
of  benzene  ?  Many  chemists  have  attempted  to  answer  this 
question,  and  several  constitutional  formulae  for  benzene  have 
been  put  forward;  that  suggested  by  Kekule  in  1865  was 
for  a  long  time  considered  to  be  the  most  satisfactory,  but 
others,  such  as  those  of  Glaus  and  Ladenburg,  also  received 
support. 


H^C 


H-C 


H-C 


8 


C— H        H— C 


C-H        H-C 


Glaus. 
(Diagonal  formula.) 


A 

Kekule. 

(Prism  formula.) 

It  will  be  seen  that  these  three  formulae  all  represent  the 
molecule  of  benzene  as  a  symmetrical  closed-chain  of  six 
carbon  atoms,  and  that  they  differ,  in  fact,  only  as  regards  the 
way  in  which  the  carbon  atoms  are  represented  as  being 
united  with  one  another ;  a  little  consideration  will  make  it 
clear,  moreover,  that  the  only  difference  between  them  lies  in 
the  manner  of  indicating  the  state  or  condition  of  the  fourth 
affinity  of  each  carbon  atom.  In  Kekule's  formula,  for 
example,  two  lines  (or  a  double  bond)  are  drawn  between 
alternate  carbon  atoms,  a  method  of  representation  which  is 
analogous  to  that  adopted  in  the  case  of  ethylene  and  other 
olefines;  in  the  formulae  of  Glaus  and  Ladenburg,  on  the 
other  hand,  each  carbon  atom  is  represented  as  directly  united 


AND   CONSTITUTION    OF    BENZENE.  307 

with    three   others   (but  with    a   different   three   in  the  two 
cases). 

As  it  would  be  impossible  to  enter  here  into  a  discussion  of 
the  relative  merits  of  the  above  three  formulae,  it  may  at  once 
be  stated  that  they  are  all  to  some  extent  unsatisfactory,  as 
they  do  not  account  for  certain  facts  which  have  been 
established  by  Baeyer  during  an  extended  study  of  benzene 
derivatives.  In  order  to  meet  these  objections,  it  has  recently 
been  suggested  by  Armstrong,  and  shortly  afterwards  by 
Baeyer,  that  the  constitution  of  benzene  may  be  best  repre- 
sented by  the  formula 

H 

i 


Armstrong  (Centric  formula). 

which,  although  in  the  main  similar  to  those  given  above, 
especially  to  that  of  Glaus,  differs  from  them  all  in  this  : 
The  fourth  affinity  of  each  of  the  six  carbon  atoms  is  repre- 
sented as  directed  towards  a  centre  (as  shown  by  the  short 
lines)  in  order  to  indicate  that,  by  the  mutual  action  of  the 
six  affinities,  the  power  of  each  is  exhausted  or  rendered 
latent,  without  bringing  about  actual  union  with  another 
carbon  atom.  This  formula,  named  by  Baeyer  the  centric 
formula,  accounts  for  all  facts  relating  to  benzene  and  its 
derivatives,  at  least  as  well  as,  and  in  some  respects  better 
than  any  which  has  yet  been  advanced,  and  its  very  indefinite- 
ness  must  be  regarded  as  a  point  in  its  favour  ;  it  is,  therefore, 
generally  adopted  at  the  present  time. 

It  now  becomes  necessary  to  give  at  greater  length  a  few 
of  the  more  important  arguments  which,  in  addition  to  those 
already  considered,  have  led  to  the  conclusion  that  the 
molecule  of  benzene  consists  of  a  symmetrical  closed-chain 


308  MANUFACTURE,    PURIFICATION,    PROPERTIES, 

of  six  carbon  atoms,  each  of  which  is  united  with  one  atom 
of  hydrogen;  also  to  point  out  how  simply  and  accurately 
this  view  of  its  constitution  accounts  for  a  number  of  facts, 
relating  to  benzene  and  its  derivatives,  which  would  other- 
wise be  incapable  of  explanation. 

In  the  first  place,  then,  it  may  be  repeated  that  benzene 
is  a  very  stable  substance ;  although  it  is  readily  acted  on  by 
powerful  chemical  agents,  such  as  nitric  acid,  sulphuric  acid, 
and  bromine,  and  thereby  converted  into  new  compounds, 
all  these  products  or  derivatives  of  benzene  contain  six  carbon 
atoms ;  the  hydrogen  atoms  may  be  displaced  by  certain  atoms 
or  groups,  and  these,  in  their  turn,  may  be  displaced  by  others, 
but  in  spite  of  all  these  changes,  the  six  atoms  of  carbon 
remain,  forming,  as  it  were,  a  stable  and  permanent  nucleus. 
This  is  expressed  in  the  formula  by  the  closed-chain  of  six 
carbon  atoms,  all  of  which  are  represented  in  the  same  state 
of  combination,  which  implies  that  there  is  no  reason  why 
one  should  be  attacked  and  taken  away  more  readily  than 
another. 

Again,  a  great  many  compounds,  which  may  be  prepared 
from,  and  converted  into,  benzene,  contain  more  than  six  atoms 
of  carbon ;  when,  however,  such  compounds  are  treated  in  a 
suitable  manner,  they  are  easily  converted  into  substances 
containing  six,  but  not  less  than  six  atoms  of  carbon.  This 
fact  shows  that  in  these  benzene  derivatives  there  are  six 
atoms  of  carbon  which  are  in  some  way  different  from  the 
others,  and  this  is  also  accounted  for  by  assuming  the 
existence  of  the  stable  nucleus ;  the  additional  carbon  atoms, 
not  forming  part  of,  but  being  simply  united  with,  this 
nucleus,  are  more  easily  attacked  and  removed. 

Further,  it  will  be  remembered  that  although  benzene 
usually  gives  substitution  products,  it  is  capable,  under 
certain  conditions,  of  forming  additive  products  of  the  type 
C6H6X6 ;  this  behaviour  is  also  accounted  for,  since,  in  the 
formula,  only  three  of  the  four  affinities  of  each  carbon  atom 
are  represented  as  actively  engaged,  and  each  carbon  atom  is 


AND   CONSTITUTION   OP    BENZENE.  309 

therefore  capable  of  combining  directly  with  one  monovalent 
atom  or  group,  so  as  to  form  finally  a  fully  saturated  com- 
pound of  the  type, 


When  benzene  is  partially  reduced  and  converted  into  a  di-  or 
tetra-additive  derivative,  the  compounds  obtained  differ  very  much 
from  the  original  hydrocarbon,  the  difference  being,  in  fact,  much 
the  same  as  that  which  exists  between  saturated  and  nnsaturated 
compounds  ;  in  other  words,  when  benzene  or  a  derivative  of  benz- 
ene combines  with  two  or  four  monad  atoms,  the  product  is  no 
longer  characterised  by  great  stability,  but  shows  the  ordinary 
behaviour  of  unsaturated  compounds,  inasmuch  as  it  is  readily 
oxidised  and  readily  combines  with  bromine. 

Dihydrobenzene,  C6H8,  and  tetrahydrobenzene,  C6H10,  combine 
directly  with  bromine  at  ordinary  temperatures  to  form  the  com- 
pounds C6H8Br4  and  C6H]0Br2  respectively,  just  as  ethylene  under 
similar  conditions  yields  ethylene  dibromide.  These  facts  are 
accounted  for  by  assuming  that,  whenever  benzene  and  its  derivatives 
are  converted  into  di-  and  tetra-additive  compounds,  the  symmetry 
of  the  molecule  is  disturbed  ;  two  or  four  of  the  six  carbon  affinities 
(represented  in  the  centric  formula  by  the  short  lines  directed 
towards  the  centre)  being  now  occupied  in  combining  with  the 
additive  atoms,  the  remainder  are  released  from  their  original  state 
of  combination,  and  become  united  in  the  same  way  as  in  ethylene  ; 
di-  and  tetra-hydrobenzene,  for  example,  may  be  represented  by  the 
formulae 


Dihydrobenzene,  or  Tetrahydrobenzene,  or 

benzene  dihydride-  benzene  tetrahydride. 


310  ISOMER1SM    OF    BENZENE    DERIVATIVES. 


CHAPTEE    XVIII. 

ISOMERISM    OF    BENZENE    DERIVATIVES,    AND    DETERMINATION 
OF    THEIR    CONSTITUTION. 

The  most  convincing  evidence  that  the  molecule  of  benzene 
is  symmetrical  is  derived  from  a  study  of  the  isomerism  of 
benzene  derivatives.  It  has  been  proved,  in  the  first  place, 
that  it  is  possible  to  substitute  1,  2,  3,  4,  5,  or  6  monovalent 
atoms  or  groups  for  a  corresponding  number. of  the  hydrogen 
atoms  in  benzene,  compounds  such  as  bromobenzene,  C6H5Br, 
dinitrobenzene,  C6H4(N02)2,  trimethylbenzene,  C6H3(CH3)3, 
tetrachlorobenzene,  CgH^Cl^  pentamethylbenzene,  C6H(CH3)5, 
and  hexacarboxybenzene,  Cg(COOH)6,  being  produced ;  the 
substituting  atoms  or  groups  may,  moreover,  be  identical  or 
dissimilar. 

An  examination  of  such  substitution  products  of  benzene 
has  shown  that  when  only  one  atom  of  hydrogen  is  displaced 
by  any  given  atom  or  group,  the  same  compound  is  always 
produced — that  is  to  say,  the  mono-substitution  products  of 
benzene  exist  only  in  one  form  ;  when,  for  example,  one  atom 
of  hydrogen  is  displaced  by  a  nitro-group,  no  matter  in  what 
way  this  change  may  be  brought  about,  the  same  substance, 
nitrobenzene,  C6H5-NO2,  is  always  produced. 

The  only  conclusion  to  be  drawn  from  this  fact  is  that  the 
molecule  of  benzene  is  symmetrical ;  if  it  were  not,  but  were 
represented  by  any  formula,  such  as 

(a)  H— C\  /C— H  (a) 

llX-ocj 

(a)  H— (Y    |       |  XC— H  (a) 
H    H 

(b)  (b) 

it  would  be  possible,  by  displacing  one  atom  of  hydrogen,  to 
obtain  (at  least)  two  isomeric  products ;  one  by  displacing  one 


ISOMERISM  $5&  BENZENE    DERIVATIVES.  311 


of  the  (a),  another  by  displacing  one  of  the  (6),  hydrogen 
atoms. 

The  existence  of  the  mono-substitution  products  of  benzene 
in  one  form  only,  might,  of  course,  be  explained  by  assuming 
that  one  particular  hydrogen  atom  was  always  displaced  first ; 
when,  for  example,  acetic  acid  is  treated  with  soda,  only 
one  of  the  four  hydrogen  atoms  is  displaceable,  and  con- 
sequently the  same  salt  is  invariably  produced.  In  the  case 
of  benzene,  however,  it  has  been  shown  that  the  same  sub- 
stance is  formed  no  matter  which  of  the  six  hydrogen  atoms 
is  displaced ;  therefore  they  are  all  in  the  same  state  of  com- 
bination. 

The  manner  in  which  this  has  been  clone  may  be  indicated  by  the 
following  example :  Phenol,  C6H5-OH,  or  hydroxybenzene,  ob- 
tained indirectly  by  displacing  one  atom  of  hydrogen  (A)  by  the 
hydroxyl-group,  may,  with  the  aid  of  phosphorus  pentabromide, 
l)e  directly  converted  into  bromobenzene,  C6H5Br,  and  the  latter 
may  be  transformed  into  benzoic  acid  (or  carboxybenzene), 
C6H5-COOH,  by  submitting  it  to  the  action  of  sodium  and  carbon 
dioxide  ;  as  these  three  substances  are  produced  from  one  another 
by  simple  interactions,  there  is  every  reason  to  suppose  that  the 
carboxyt-group  in  benzoic  acid  is  united  with  the  same  carbon 
atom  as  the  bromine  atom  in  bromobenzene  and  the  hydroxyl- 
group  in  phenol ;  that  is  to  say,  that  the  same  hydrogen  atom  (A) 
has  been  displaced  in  all  three  cases.  Now  the  benzoic  acid 
obtained  in  this  way  may  be  converted  into  three  different 
hydroxy benzoic  acids  of  the  composition  C6H4(OH)-COOH,  the  differ- 
ence between  them  being  due  to  the  fact  that  the  hydroxyl-group 
has  displaced  a  different  hydrogen  atom  (B.C.D. )  in  each  case; 
each  of  these  hydroxybenzoic  acids  forms  a  calcium  salt  which 
yields  phenol  on  distillation  (the  carboxyl-group  being  displaced 
by  hydrogen),  and  the  three  specimens  of  phenol  thus  produced  are 
identical  with  the  original  phenol ;  it  is  evident,  therefore,  that  at 
least  four  (A. B.C.D.)  hydrogen  atoms  in  benzene  are  in  the  same 
state  of  combination,  and  occupy  the  same  relative  position  in  the 
molecule ;  in  a  similar  manner  it  can  be  shown  that  this  is  true  of 
all  six. 

By  substituting  two  monovalent  atoms  or  groups  for  two 
of  the  atoms  of  hydrogen  in  benzene,  three,  but  not  more 
than  three  substances  having  different  properties  are  obtained  ; 


312  tSOMERISM   OF   BENZENE   DERIVATIVES. 

there  are,  for  example,  three  dinitrobenzenes,  C6H4(N02)2, 
three  dibromobenzenes,  C6H4Br2,  three  dihydroxybenzenes, 
C6H4(OH)2,  three  nitrohydroxybenzenes,  C6H4(N02)-OH,  and 
so  on. 

Three  isomerides  are  not  always  produced  in  any  particular 
reaction,  and  all  di-substitution  products  of  benzene  are  not  known 
to  exist  in  three  forms ;  but  from  the  study  of  a  great  many  com- 
pounds of  this  kind,  it  is  practically  certain  that  they  all  could  be 
obtained  in  three  isomeric  modifications. 

Now  the  existence  of  these  three  isomerides  can  be 
accounted  for  in  a  very  simple  manner  with  the  aid  of  the 
formula  already  given,  which,  for  this  purpose,  may  con- 
veniently be  represented  by  a  simple  hexagon,  numbered  as 
shown,  the  symbols  C  and  H  being  omitted  for  the  sake  of 
simplicity. 


Suppose  that  any  mono-substitution  product,  C6H5X,  which, 
as  already  stated,  exists  only  in  one  form,  be  converted  into  a 
di-substitution  product,  C6H4X2 ;  then  if  it  be  assumed  that 
the  atom  or  group  (X)  first  introduced  occupied  any  given 
position,  say  that  numbered  1,  the  second  atom  or  group  mny 
have  substituted  any  one  of  the  hydrogen  atoms  at  2,  3,  4,  5, 
or  6,  giving  a  substance,  the  constitution  of  which  might  be 
represented  by  one  of  the  following  five  formulae : 


i 

I.  II.  III.  IV.  V. 

These  five  formulae,  however,  represent  three  isomeric  sub- 
stances, and  three  only.  The  formula  (iv.)  represents  a  com- 
pound in  which  the  several  atoms  occupy  the  same  relative 


ISOMERISM    OF    BENZENE    DERIVATIVES.  313 

positions  as  in  the  substance  represented  by  the  formula  (11.), 
and  for  the  same  reason  the  formula  (v.)  is  identical  with 
(i.).  Although  there  is  at  first  sight  an  apparent  difference,  a 
little  consideration  will  show  that  this  is  simply  due  to  the 
fact  that  the  formula  are  viewed  from  one  point  only ;  if  the 
formulas  iv.  and  v.  be  held  before  a  mirror,  or  viewed  through 
the  paper,  it  will  be  seen  at  once  that  they  are  identical 
with  n.  and  i.  respectively.  Each  of  the  formulae  I.,  11.,  and 
in.,  on  the  other  hand,  represents  a  different  substance,  because 
in  no  two  cases  are  all  the  atoms  in  the  same  relative  posi- 
tions ;  in  other  words,  the  di-substitution  products  of  benzene 
exist  theoretically  in  three  isomeric  forms. 

In  the  foregoing  examples  the  two  substituting  atoms  or 
groups  have  been  considered  to  be  identical,  but  even  when 
they  are  different,  experience  has  shown  that  only  three  di- 
substitution  products  can  be  obtained,  and  this  fact,  again, 
is  in  accordance  with  the  theory.  If  in  the  above  five 
formulae  either  of  the  X's  be  written  Y  to  express  a  difference 
in  the  substituting  groups,  it  will  be  seen  that,  as  before,  the 
formula  i.  is  identical  with  v.,  and  n.  with  iv.,  but  that  i.,  n., 
and  in.  all  represent  different  arrangements  of  the  atoms — 
that  is  to  say,  three  different  substances. 

Since  the  di-substitution  products  of  benzene  exist  in  three 
isomeric  forms,  it  is  convenient  to  have  some  way  of  dis- 
tinguishing them  by  name  ;  for  this  reason  all  di-substitution 
products  which  are  found  to  have  the  constitution  repre- 
sented by  formula  i.  are  called  ortho-compounds,  and  the 
substituting  atoms  or  groups  are  said  to  be  in  the  ortho-  or 
1 : 2-position  to  one  another ;  those  substances  which  may  be 
represented  by  the  formula  n.  are  termed  meta-compounds, 
and  the  substituting  atoms  or  groups  are  spoken  of  as  occupy- 
ing the  meta-  or  1 : 3-position ;  the  term  para  is  applied  to 
compounds  represented  by  the  formula  in.,  in  which  the 
atoms  or  groups  are  situated  in  the  para-  or  1 : 4-position. 

Ortho-compounds,  then,  are  those  in  which  it  is  assumed, 
for  reasons  given  below,  that  the  two  substituting  atoms  or 


314  ISOMERISM    OF    BENZENE    DERIVATIVES. 

groups  are  combined  with  carbon  atoms  which  are  themselves 
directly  united;  instead  of  expressing  the  constitution  of  any 
ortho-compound  by  the  formula  r.,  and  representing  the 
substituting  atoms  or  groups  as  combined  with  the  carbon 
atoms  1  and  2,  it  would  therefore  be  just  the  same  if  they 
were  represented  as  united  with  the  carbon  atoms  2  and  3,  3 
and  4,  4  and  5,  5  and  6,  or  6  and  1 ;  the  arrangement  of  all 
the  atoms  would  be  the  same,  because  the  benzene  molecule 
is  symmetrical,  and  the  numbering  of  the  carbon  atoms 
simply  a  matter  of  convenience.  In  a  similar  manner  the 
substituting  atoms  or  groups  in  meta-compounds  may  be 
represented  as  combined  with  any  two  carbon  atoms  which 
are  themselves  not  directly  united,  but  linked  together  by  one 
carbon  atom ;  it  is  quite  immaterial  which  two  carbon  atoms 
are  chosen,  since  atoms  or  groups  occupying  the  1:3,  2:4,  3:5, 
4:6,  or  5:l-position  are  identically  situated  with  regard  to  all 
the  other  atoms  of  the  molecule.  For  the  same  reason  para- 
compounds  may  be  represented  by  placing  the  substituting 
atoms  or  groups  in  the  1:4,  2:5,  or  3:6-position. 

When  more  than  two  atoms  of  hydrogen  in  benzene  are 
substituted,  it  has  been  found  that  the  number  of  isomerides 
differs  according  as  the  substituting  atoms  or  groups  are 
identical  or  not.  By  displacing  three  atoms  of  hydrogen  by 
three  identical  atoms  or  groups,  three  isomerides  can  be 
obtained,  three  trimethylbenzenes,  C6H3(CH3)3,  for  example, 
being  known.  Again,  the  existence  of  these  isomerides  can  be 
easily  accounted  for,  since  their  constitutions  may  be  repre- 
sented as  follows  : 


—  X 


X 
Adjacent.  Asymmetrical.  Symmetrical. 

No  matter  in  what  other  positions  the  substituting  atoms  or 


ISOMERISM    OP    BENZENE    DERIVATIVES.  315 

groups  be  placed,  it  will  be  found  that  the  arrangement  is 
the  same  as  that  represented  by  one  of  the  above  formulas ; 
the  position  1:2:3,  for  example,  is  identical  with  2:3:4,  3:4:5, 
&c. ;  1:3:4  with  2:4:5,  3:5:6,  &c.,  and  1:3:5  with  2:4:6.  For 
the  purpose  of  referring  to  such  tri-substitution  products,  the 
terms  given  above  are  often  employed. 

The  tetra-substitution  products  of  benzene,  in  which  all 
the  substituting  atoms  or  groups  are  identical,  also  exist  in 
three  isomeric  forms  represented  by  the  following  formulae  : 

XXX 
-X 

-X         X— ^  >-X          X- 

i" 

When,  however,  five  or  six  atoms  of  hydrogen  are  displaced  by 
identical  atoms  or  groups,  only  one  substance  is  produced. 

When  more  than  two  atoms  of  hydrogen  are  displaced  by  atoms 
or  groups  which  are  not  all  identical,  the  number  of  isomerides 
which  can  be  obtained  is  very  considerable ;  in  the  case  of  any 
tri-substitution  product,  C6H3X2Y,  for  example,  six  isomerides 
might  be  formed,  as  may  be  easily  seen  by  assigning  a  definite 
position,  say  1,  to  Y ;  the  isomerides  would  then  be  represented  by 
formulas  in  which  the  groups  occupied  the  position  1:2:3,  1:2:4, 
1:2:5,  1:2:6,  1:3:4,  or  1:3:5,  all  of  which  would  be  different. 

All  the  cases  of  isomerism  considered  up  to  the  present 
have  been  those  due  to  the  substituting  atoms  or  groups 
occupying  different  relative  positions  in  the  benzene  nucleus ; 
as,  however,  many  benzene  derivatives  contain  groups  of  atoms 
which  themselves  exist  in  isomeric  forms,  such  compounds 
also  exhibit  isomerism  exactly  similar  to  that  already  met 
with  in  the  case  of  the  paraffins,  alcohols,  &c.  There  are,  for 
example,  two  isomeric  hydrocarbons  of  the  composition 
C6H5-C3H7,  namely,  propylbenzene,  C6H5'CH2-CH2-CH3,  and 
isopropylbenzene,  C6H5-CH(CH3)2,  just  as  there  are  two 
isomeric  ethereal  salts  of  the  composition  C3H7I.  As, 
moreover,  the  two  propylbenzenes,  C61I--C3H7,  are  isomeric 


316  ISOMERISM    OP   BENZENE    DERIVATIVES. 

with  the  three  (ortho-,  meta-,  and  para-)  ethylmethylbenzenes, 
C6H4(C2H5)-CH3,  arid  also  with  the  three  (adjacent,  sym- 
metrical, and  asymmetrical)  trimethylbenzenes,  C6H3(CH3)3, 
there  are  in  all  eight  hydrocarbons  of  the  molecular  formula 
C9H12,  derived  from  benzene. 

In  studying  the  isomerism  of  benzene  derivatives,  the 
clearest  impressions  will  be  gained  by  invariably  making  use 
of  a  simple,  unnumbered  hexagon  to  represent  C6H6,  and  by 
expressing  the  constitutions  of  simple  substitution  products 
by  formula  such  as 

N02  CH3 

— N02 
-Cl        ^  J  ^  J        CH3-L  J-CH3 


Chlorobenzene.        Dinitrobenzene.  Nitrophenol.  Trimethylbenzene. 

The  omission  of  the  symbols  C  and  H  is  attended  by  no 
disadvantage  whatsoever,  because,  in  order  to  convert  the 
above  into  the  ordinary  molecular  formulae,  it  is  only  necessary 
to  write  C6  instead  of  the  hexagon,  and  then  to  count  the 
unoccupied  corners  of  the  hexagon  to  find  the  number  of 
hydrogen  atoms  in  the  nucleus,  the  substituting  atoms  or 
groups  being  added  afterwards.  In  the  case  of  chlorobenzene, 
for  example,  there  are  five  unoccupied  corners,  so  that  the 
molecular  formula  is  C6H5C1  ;  whereas  in  the  case  of  tri- 
methylbenzene  there  are  three,  and  the  formula,  therefore,  is 


As,  however,  such  graphic  formulae  occupy  a  great  deal  of 
space,  their  constant  use  in  a  text-book  is  out  of  the  question, 
and  other  methods  have  to  be  adopted.  The  most  usual 
course  in  the  case  of  the  di-derivatives  is  to  employ  the  terms 
ortho-,  meta-,  and  para-,  or  simply  the  letters  o,  m,  and  p,  as, 
for  example,  ortho-dinitrobenzene  or  o-dinitrobenzene,  meta- 
nitraniline  or  m-nitraniline,  para-nitrophenol  or  jp-nitrophenol  ; 
the  relative  positions  of  the  atoms  or  groups  may  also  be  ex- 


ISOMERISM    OF    BENZENE    DERIVATIVES.  317 

pressed  by  numbers;  ortho-chloronitrobenzene,  for  example,  may 

be  described  as  1 :2-chloronitrobenzene,  as  C6H4<^iSTQ  (2)>  or  as 

i      2 
C6H4C1-N02,  the  corresponding  para-compound  as  l:4-chloro- 

QJ    (1)  *      4 

nitrobenzene,  as  C6H4<^-.^  ,^,  or  as  C6H4C1-N02.       In  the 

case  of  the  tri-derivatives  the  terms  symmetrical,  asymmetrical, 
and  adjacent  (compare  p.  314)  may  be  employed  when  all  the 
atoms  or  groups  are  the  same,  but  when  they  are  different 
the  constitution  of  the  compound  is  usually  expressed  with 
the  aid  of  numbers ;  the  tribromaniline  of  the  constitution 


Br— 


Br 

1356 

for  example,  is  described  as  C6H2Br3-NH2[Br:Br:Br:lSTH2], 
or  as  C6H2Br3.NH2[3Br:NH2  =  2:4:6:1],  and  it  is  of 
course  quite  immaterial  from  which  corner  of  the  imaginary 
hexagon  the  numbering  is  commenced. 

Determination  of  the  Constitution  of  Benzene  Derivatives. 

It  has  been  pointed  out  that  the  di-substitution  products 
of  benzene,  such  as  dibromobenzene,  C6H4Br2,  dihydroxy- 
benzene,  C6H4(OH)2,  and  nitraniline,  C6H4(X02)-NH2,  exist 
in  three  isomeric  forms,  and  that  their  isomerism  is  due  to  the 
different  relative  positions  of  the  substituting  atoms  or  groups 
in  the  benzene  nucleus ;  it  is  evident,  however,  that  in  order 
to  arrive  at  the  constitution  of  any  one  of  these  substances, 
and  to  be  able  to  say  whether  it  is  an  ortho-,  meta-,  or  para- 
compound,  a  great  deal  of  additional  information  is  required. 

Now  the  methods  which  are  adopted  in  deciding  questions 
of  this  kind  at  the  present  time  are  comparatively  simple, 
but  they  are  based  on  the  results  of  work  which  has  extended 
over  many  years.  It  has  been  found,  in  the  first  place,  that 


318  ISOMERISM    OF    BENZENE    DERIVATIVES. 

a  given  di-substitution  product  of  benzene  may  be  converted 
by  more  or  less  indirect  methods  into  many  of  the  other 
di-substitution  products  of  the  same  series ;  or^o-dinitroben- 
zene,  C6H4(N02)2,  for  example,  may  be  transformed  into  o-dia- 
midobenzene,  C6H4(NH2)2,  o-dihydroxybenzene,  C6H4(OH)2, 
o-dibromobenzene,  C6H4Br2,  o-dimethylbenzene,  C6H4(CH,).7, 
and  so  on,  similar  changes  being  also  possible  in  the  case  of 
meta-  and  para-compounds.  If,  therefore,  it  can  be  ascer- 
tained to  which  series  a  given  di-substitution  product  belongs, 
the  constitution  of  other  di-substitution  products  of  this  series 
may  be  easily  determined ;  suppose,  for  example,  that  it 
could  be  proved  that  of  the  three  dinitrobenzenes,  the  com- 
pound melting  at  90°  is  a  meta-compound,  then  it  would 
necessarily  follow  that  the  diamido-,  dihydroxy-,  dibromo-,  and 
other  di-derivatives  of  benzene  obtained  from  this  particular 
dinitro-compound  by  substituting  other  atoms  or  groups  for 
the  two  nitro-groups,  must  also  be  meta-compounds ;  it  would 
also  be  known  that  the  di-derivatives  of  benzene  obtained 
from  the  other  two  dinitrobenzenes,  melting  at  118°  and  173° 
respectively,  in  a  similar  manner  must  be  either  ortho-  or 
para-compounds. 

It  was  necessary,  therefore,  in  the  first  place,  to  determine 
the  constitution  of  one  or  two  di-derivatives  of  each  series; 
these  substances  then  served  as  standards,  and  the  constitu- 
tion of  any  other  di-derivative  was  established  by  converting 
it  by  suitable  reactions  into  one  of  these  standards. 

As  an  illustration  of  the  methods  and  arguments  originally 
employed  in  the  solution  of  problems  of  this  nature,  the  case 
of  the  dicarboxy-  and  dimethyl-derivatives  of  benzene  may 
be  quoted.  Of  the  three  dicarboxybenzenes,  C6H4(COOH)2, 
one — namely,  phthalic  acid  (p.  425),  is  very  readily  converted 
into  its  anhydride,  but  all  attempts  to  prepare  the  anhydrides 
of  the  other  two  acids  (isophthalic  acid  and  terephthalic 
acid,  pp.  426,  427)  result  in  failure ;  it  is  assumed,  therefore, 
that  the  acid  which  gives  the  anhydride  is  the  o-compound, 
because,  from  a  study  of  the  behaviour  of  many  other  dicar- 


ISOMERISM    OF    BENZENE    DERIVATIVES. 


319 


boxylic  acids,  it  has  been  found  that  anhydride  formation 
takes  place  most  readily  when  the  two  carboxyl-groups  are 
severally  combined  with  two  carbon  atoms  which  are  them- 
selves directly  united,  as,  for  example,  in  the  case  of  succinic 
acid.  In  other  words,  if  the  graphic  formulae  of  succinic 
acid  and  of  the  three  dicarboxy-derivatives  of  benzene  be 
compared,  it  will  be  evident  that  in  the  o-compound  the 
relative  position  or  state  of  combination  of  the  two  carboxyl- 
groups  is  practically  the  same  as  in  succinic  acid,  but  quite 
otherwise  in  the  case  of  the  m-  and  ^-compounds. 


CH2— COOH 


:H2— COOH 


-COOH 


COOH 


For  this,  and  other  reasons  not   stated  here,  phthalic  acid 
may  be  provisionally  regarded  as  an  or/Ao-dicarboxy  benzene. 

Again,  the  hydrocarbon  mesitylene  or  trimethylbenzene, 
C6H3(CH3)3,  may  be  produced  synthetically  from  acetone 
(p.  337),  and  its  formation  in  this  way  can  be  explained  in 
a  simple  manner,  only  by  assuming  that  mesitylene  is  a 
symmetrical  trimethylbenzene  of  the  constitution  (A). 


—  CH3 


COOH 

Mesitylenic  Acid. 


-COOH 


Dimethylbenzene. 
(Isoxylene.) 


Isophthalic  Acid. 


When  this  hydrocarbon  is  carefully  oxidised,  it  yields  an  acid 
(B)  of  the  composition  C6H3(CH3)2-COOH  (by  the  conversion 
of  one  of  the  methyl-groups  into  carboxyl),  from  which  a 
dimethylbenzene,  C6H4(CH3)2  (C),  is  easily  obtained  by  the 


320  ISOMERISM    OF    BENZENE    DERIVATIVES. 

substitution  of  hydrogen  for  the  carboxyl-group.  This  di- 
methylbenzene,  therefore,  is  a  we/a-cornpound,  because  no 
matter  which  of  the  original  three  methyl-groups  in  mesityl- 
ene  has  been  finally  displaced  by  hydrogen,  the  remaining 
two  must  occupy  the  ?ft-position.  Now  when  this  dimethyl- 
benzene  is  oxidised  with  chromic  acid,  it  is  converted  into  a 
dicarboxylic  acid  (D) — namely,  isophthalic  acid,  C6H4(COOH)2, 
which,  therefore,  must  also  be  regarded  as  a  meta-compound ; 
the  constitution  of  two  of  the  three  isorneric  dicarboxy-deriva- 
tives  of  benzene  having  been  thus  determined,  the  third — 
namely,  terephthalic  acid,  can  only  be  the  jpara-compound. 

It  is  now  a  comparatively  simple  matter  to  ascertain  to 
which  series  any  of  the  three  dimethylbenzenes  belongs; 
one  of  them  having  been  found  to  bs  the  meta-compound, 
all  that  is  necessary  is  to  submit  each  of  the  other  two  to 
oxidation,  and  that  which  gives  phthalic  acid  will  be  the 
ortho-compound,  whilst  that  which  yields  terephthalic  acid 
will  be  the  para-derivative.  Moreover,  the  constitution  of 
any  other  di-substitution  product  of  benzene  may  now  be 
determined  without  difficulty,  provided  that  it  is  possible 
to  convert  it  into  one  of  these  standards  by  simple  reactions. 

As  the  methods  which  have  just  been  indicated  are  based 
entirely  on  arguments  drawn  from  analogy,  or  from  deductions 
as  to  the  probable  course  of  certain  reactions,  the  conclusions 
to  which  they  lead  cannot  be  accepted  without  reserve ;  there 
are,  however,  several  other  ways  in  which  it  is  possible  to 
distinguish  with  much  greater  certainty  between  ortho-,  meta-, 
and  para-compounds,  and  of  these  that  employed  by  Kb'rner 
may  be  given  as  an  example. 

Korner's  method  is  based  on  the  fact  that,  if  any  di-sub- 
stitution product  of  benzene  be  converted  into  a  tri-derivative 
by  further  displacement  of  hydrogen  of  the  nucleus,  the 
number  of  isomerides  which  may  be  obtained  from  an  ortho-, 
meta-,  and  para-compound  is  different  in  the  three  cases,  so  that 
by  ascertaining  the  number  of  these  products  the  constitution 
of  the  original  di-derivative  may  be  established.  Suppose, 


ISOMERISM    OF    BENZENE    DERIVATIVES. 


321 


for  example,  that  one  of  the  three  isomeric  dibromobenzenes 
be  converted  into  nitrodibromobenzene  by  treatment  with 
nitric  acid ;  then,  if  it  be  the  or^o-dibromo-compound,  it 
is  possible  to  obtain  from  it  two,  but  only  two,  nitrodibromo- 
beiizenes,  because,  although  there  are  four  hydrogen  atoms, 
any  one  of  which  may  be  displaced  by  a  nitro-group,  as 
represented  by  the  following  formulas, 

Br  Br 


-Br 


-N02 


-Br 


NO.,— 


-Br    N02— 


-Br 


I.  II.  III.  IV. 

the  compound  of  the  constitution  (in.)  is  identical  with  (n.), 
and  (iv.)  with  (i.),  the  relative  positions  of  all  the  atoms 
being  the  same  in  the  two  cases  respectively. 

If,  on  the  other  hand,  the  dibromobenzene  be  the  meta-com- 
pouiid,  it  might  yield  three,  and  only  three,  isomeric  nitro- 
derivatives,  which  would  be  represented  by  the  first  three  of  the 
following  formulae,  the  fourth  being  identical  with  the  second : 

Br  Br  Br 


-Br   X02— 


-Br 


-Br 


Finally,    if    the    substance   in    question  be  j9«ra-dibromo- 
benzene,  it  could  give  only  one  nitro-derivative,  the  following 


four  formulae  being  identical  : 


Br 


Br 


— XOo 


It  is  obvious,  then,  that  this  method  may  be  applied  in 

u 


322  ISOMERISM    OF    BENZENE    DERIVATIVES. 

ascertaining  to  which  series  any  di-substitution  product 
belongs  ;  it  may  also  be  employed  in  determining  the  con- 
stitution of  the  tri-derivatives  in  a  similar  manner. 

At  the  present  time,  therefore,  the  constitution  of  any  new 
benzene  derivative  is,  as  a  rule,  very  easily  ascertained ;  it  is 
simply  converted  into  some  compound  of  known  constitution, 
or  the  number  of  isomerides  obtained  from  it  by  substitution 
is  determined. 


CHAPTER    XIX. 

GENERAL  PROPERTIES  OF  AROMATIC  COMPOUNDS. 

Classification  of  Organic  Compounds. — The  examples  given 
in  the  foregoing  pages  will  have  afforded  some  indication  of 
the  large  number  of  compounds  which  it  is  possible  to  prepare 
from  benzener  by  the  substitution  of  various  elements  or 
groups  for  atoms  of  hydrogen ;  as  the  substances  formed  in 
this  way,  and  many  other  benzene  derivatives  which  occur  in 
nature,  or  may  be  prepared  synthetically,  still  retain  much  of 
the  characteristic  chemical  behaviour  of  benzene,  and  differ 
in  many  respects  from  the  paraffins,  alcohols,  acids,  and  all 
other  compounds  previously  considered  (part  i.),  it  is  con- 
venient to  class  benzene  and  its  derivatives  in  a  separate 
group. 

Organic  compounds  are  therefore  classed  in  two  principal 
divisions,  the  fatty  and  the  aromatic.  The  word  'fatty/ 
originally  applied  to  some  of  the.  acids  of  the  C71H2R02  series 
(part  i.  p.  142),  is  now  used  to  denote  all  compounds  which 
may  be  considered  as  derivatives  of  marsh-gas,  and  which 
cannot  be  regarded  as  directly  derived  from  benzene ;  all  the 
compounds  described  in  part  i.  belong  to  the  fatty  group  or 
division.  Benzene  and  its  derivatives,  on  the  other  hand, 
are  classed  in  the  '  aromatic '  group,  this  term  having  been 
first  applied  to  certain  naturally  occurring  compounds  (which 


GENERAL    PROPERTIES    OF    AROMATIC    COMPOUNDS.          323 

have  since  been  proved  to  be  benzene  derivatives)  on  account 
of  their  peculiar  aromatic  odour. 

The  fundamental  distinction  between  fatty  and  aromatic 
compounds  is  one  of  constitution.  The  reasons  which  have 
led  to  the  conclusion  that  benzene  contains  a  closed  chain  of 
six  carbon  atoms  being  equally 'valid  in  the  case  of  its  deri- 
vatives, it  is  assumed  that  this  (or  a  similar)  nucleus' is 'present 
in  all  aromatic  compounds.  The  constitution  of  a  fatty  com- 
pound, however,  is  almost  invariably  expressed  by  a  formula 
such  as  CH3-CH2.CH2.CH3,  CH2(OH).CH(OH).CH2(OH),and 
COOH.CH2-CH2-COOH,  in  which  the  carbon  atoms  do  not 
form  a  closed-,  but  an  open-chain ;  *  such  compounds,  more- 
over, may  be  regarded  as  derived  from  marsh-gas  by  a  series 
of  simple  steps.  For  these  reasons,  compounds  belonging  to 
the  fatty  series  are  often  spoken  of  as  open-chain  compounds, 
in  contradistinction  to  the  closed-chain  compounds  of  the 
aromatic  group. 

It  must  not,  however,  be  supposed  that  all  aromatic  are 
sharply  distinguished  in  any  way  from  all  fatty  compounds, 
or  that  either  class  can  be  defined  in  any  exact  terms.  Many 
compounds,  the  constitutions  of  which  must  be  represented 
by  closed-chain  formula?,  are  nevertheless  placed  in  the  fatty 
group,  simply  because  to  class  them  in  the  aromatic  division 
would  remove  them  from  those  substances  to  which  they  are 
most  closely  related  ;  succinimide  (part  i.  p.  237),  for  example, 
is  a  closed-chain  compound  in  the  strict  sense  of  the  word, 
but  is  clearly  more  conveniently  considered  in  the  fatty  series, 
because  of  its  relationship  to  succinic  acid.  Although,  again, 
the  members  of  the  aromatic  group  may  all  be  regarded  as 
derivatives  of  benzene,  they  may  also  be  considered  as  derived 
from  marsh-gas,  since  not  only  benzene  itself,  but  many  other 
aromatic  compounds,  may  be  directly  obtained  from  members 

*  The  terms  'open-chain'  and  'closed-chain'  originated  in  the  chain-like 
appearance  of  the  graphic  formulae  as  usually  written,  and  are  not  intended 
to  convey  the  idea  that  the  atoms  are  joined  together  by  any  form  of 
matter,  or  that  they  are  all  arranged  in  straight  lines. 


324          GENERAL    PROPERTIES    OF    AROMATIC    COMPOUNDS. 

of  the  fatty  series  by  simple  reactions,  and,  conversely,  many 
aromatic  compounds  may  be  converted  into  those  of  the  fatty 
series. 

Some  examples  of  the  production  of  aromatic  from  fatty 
compounds  have  already  been  given — namely,  the  formation 
of  benzene  by  the  polymerisation  of  acetylene,  and  that 
of  mesitylene  by  the  condensation  of  acetone ;  these  two 
changes  may  be  expressed  graphically  in  the  following 
manner : 


C-H 

*        III 

H— €• 


r          r 


CO-CH3 
CH3  CH 

and  may  be  regarded  as  typical  reactions,  because  many  other 
substances,  similar  in  constitution  to  acetylene  and  acetone 
respectively,  may  be  caused  to  undergo  analogous  transforma- 
tions. Bromacetylene,  CBriCH,  for  example,  may  be  con- 
verted into  (symmetrical)  tribromobenzene,  simply  by  leav- 
ing it  exposed  to  direct  sunlight, 

3C2HBr  =  C6H8Brs; 

and  methylethyl  ketone  (a  homologue  of  acetone)  is  trans- 
formed into  symmetrical  triethylbenzene  (a  homologue  of 
mesitylene)  by  distilling  it  with  sulphuric  acid, 

3CH8.CO.C2H6  =  C6H8(C2H6)8  +  3H20. 

General  Character  of  Aromatic  Compounds. — Although, 
then,  it  is  impossible  to  draw  any  sharp  line  between  fatty  and 


GENERAL   PROPERTIES   OF    AROMATIC   COMPOUNDS.          325 

aromatic  compounds,  and  many  substances  are  known  which 
form  a  connecting  link  between  the  two  divisions,  the  great 
majority  of  aromatic  substances  differ  materially  from  those 
of  the  fatty  division  in  constitution,  and  consequently  also  in 
properties. 

Speaking  generally,  aromatic  compounds  contain  a  larger 
percentage  of  carbon  than  those  of  the  fatty  division,  and 
probably  for  this  reason,  they  are  more  frequently  crystalline 
at  ordinary  temperatures.  They  are,  as  a  rule,  less  readily 
resolved  into  simple  substances  than  are  the  members  of  the 
fatty  series,  although  in  most  cases  they  are  more  easily  con- 
verted into  substitution  products.  Their  behaviour  with 
nitric  acid  and  with  sulphuric  acid  is  very  characteristic,  and 
distinguishes  them  from  nearly  all  fatty  compounds,  inas- 
much as  they  are,  as  a  rule,  readily  converted  into  nitro-  and 
sulphonic-derivatives  respectively  by  the  displacement  of 
hydrogen  atoms  of  the  nucleus, 

COOTT 
C6H5-COOH  +  HN08  -   CflH4<NO       +  H20 

C6H6-OH  +  3HN08    '=   C6H2(OH)(N02)3 
C.H..NH,  +  H2S04       =   C 


Fatty  compounds  rarely  give  sulphonic-  or  nitro-deriva- 
tives  under  the  same  conditions,  but  are  acted  on  in  such 
a  way  that  they  are  resolved  into  two  or  more  simpler 
substances. 

When  aromatic  nitro-compounds  are  treated  with  reducing 
agents,  they  are  converted  into  amido-compounds, 

C6H5.NO,  +  6H  -  C6H5-NH9  +  2H20 
C6H4(N02)2  +  12H  =  C6H4(NH2)2  +~4H20. 

These  amido-compounds  differ  from  the  fatty  amines  in  at 
least  one  very  important  respect,  inasmuch,  as  they  are  con- 
verted into  diazo-r.orn  pounds  (p.  370)  on  treatment  with  nitrous 
acid  in  the  cold;  this  behaviour  is  highly  clinracteristic,  and 


326         GENERAL    PROPERTIES    OP    AROMATIC    COMPOUNDS. 

the  diazo-compounds  form  one   of  the  most  interesting  and 
important  classes  of  aromatic  substances. 

It  has  already  been  pointed  out  that  benzene  does  not 
show  the  ordinary  behaviour  of  unsaturated  fatty  compounds, 
although  under  certain  conditions  both  the  hydrocarbon  and 
its  derivatives  are  capable  of  forming  additive  compounds  by 
direct  combination  with  two,  four,  or  six  (but  not  with  one, 
three,  or  five)  monovalent  atoms.  This  fact  proves  that 
benzene  is  not  really  a  saturated  compound  like  methane,  or 
ethane,  for  example,  both  of  which  are  quite  incapable  of 
yielding  derivatives  except  by  substitution.  Nevertheless, 
the  conversion  of  benzene  and  its  derivatives  into  additive 
products,  is,  as  a  rule,  much  less  readily  accomplished  than  in 
the  case  of  fatty,  unsaturated  compounds ;  the  halogen  acids, 
for  example,  which  unite  directly  with  so  many  unsaturated 
fatty  compounds,  have  no  such  action  on  benzene  and  its 
derivatives,  and  even  in  the  case  of  the  halogens  and  nascent 
hydrogen,  direct  combination  occurs  only  under  particular 
conditions.  The  compounds,  such  as  dihydrobenzene,  C6Hg, 
tetrahydrobenzene,  C6H10,  benzene  hexachloride,  C6H6C10,  and 
benzene  hexahydride,  C6H12  (hexamethylene),  obtained  in  this 
way,  have  not  yet  been  very  fully  investigated,  but  from 
what  is  known  of  their  properties,  they  form  a  connecting 
link  between  the  members  of  the  aromatic  and  fatty 
divisions  (compare  p.  309). 

When  the  hydrogen  atoms  in  benzene  are  displaced  by 
groups  or  radicles  which  are  composed  of  several  atoms, 
these  groups  are  spoken  of  as  side-chains;  ethylbenzene, 
C6H5.CH2.CH3,  benzyl  alcohol,  C6H5-CH.2.OH,  and  methyl 
aniline,  C6H5-ISTH-CH3,  for  example,  would  each  be  said  to 
contain  a  side-chain,  whereas  the  term  would  not,  as  a  rule, 
be  applied  in  the  case  of  phenol,  C6H5-OH,  nitrobenzene, 
C6H5-N02,  &c.,  where  the  substituting  groups  are  com- 
paratively simple,  and  do  not  contain  carbon  atoms. 

Now  the  character  of  any  particular  atom  or  group  in  the 
side-chain,  although  influenced  to  some  extent  by  the  fact 


GBNEKAL    PROPERTIES    OB'    AROMATIC    COMPOUNDS.          327 

that  the  group  is  united  with  the  benzene  nucleus,  is  on  the 
whole  very  similar  to  that  which  it  possesses  in  fatty  com- 
pounds. The  consequence  is  that  aromatic  compounds  con- 
taining side-chains  of  this  kind  have  not  only  the  properties 
already  referred  to,  as  characteristic  of  the  derivatives  of 
benzene,  but  show  also,  to  a  certain  extent,  the  behaviour 
of  fatty  compounds.  Benzyl  chloride,  C6H5-CH2C1,  for  ex- 
ample, may  be  directly  converted  into  the  nitro-derivative, 
CflH4(N02).CH2Cl,  and  the  sulphonic  acid,  C6H4(S03H>CH2C1, 
reactions  characteristic  of  aromatic  compounds ;  on  the 
other  hand,  the  -CH2C1  group  may  be  transformed  into 
-CH2-OH,  -CHO,  -COOH,  and  so  on,  just  as  may  the  same 
group  in  ethyl  chloride,  CH3-CH2C1,  and  similar  fatty  com- 
pounds, and  in  all  cases  the  products  retain,  to  some  extent, 
the  properties  of  fatty  substances  as  long  as  the  side-chain 
remains.  The  groups  forming  the  side-chains,  however,  are 
more  easily  attacked  and  removed  than  the  closed-chain 
or  nucleus;  when  ethylbenzene,  C6H5-CH2-CH3,  or  propyl- 
benzene,  C6H5-CH2-CH2-CH3,  for  example,  is  boiled  with 
chromic  acid,  the  side-chain  undergoes  oxidation,  carbon 
dioxide  is  evolved,  and  benzoic  acid,  C6H5-COOH,  is  pro- 
duced in  both  cases,  the  six  atoms  of  carbon  in  the  nucleus 
being  unchanged  (p.  417). 

Although  the  compounds  derived  from  benzene  by  direct 
substitution  are  very  numerous,  the  aromatic  group  also 
contains  a  great  many  other  substances  which  are  more 
distantly  related  to  benzene,  and  which  can  only  be  re- 
garded as  derived  from  it  indirectly.  The  hydrocarbon 
drphenylt  C6H5-C6H5,  for  example,  which,  theoretically, 
is  formed  by  the  union  of  two  phenyl  or  C6H5-  groups, 
just  as  dimethyl  or  ethane,  CH3-CH3,  is  produced  by  the 
combination  of  two  methyl-groups,  is  an  important  member 
of  the  aromatic  division,  and,  like  benzene,  is  capable  of 
yielding  a  very  large  number  of  substitution  products. 
Other  hydrocarbons  are  known  in  which  the  presence  of  two 
or  more  closed  carbon  chains,  combined  in  different  ways, 


328         GENERAL   PROPERTIES   Of   AROMATIC   COMPOUNDS. 

must  be  assumed,  as,  for  example,  in  the  cases  of  naphthalene 
(p.  442)  and  anthracene  (p.  437), 


Naphthalene.  Anthracene. 

and  there  are  also  substances,  such  as  pyridine  (p.  472)  and 
quinoline  (p.  480),  in  which  a  nitrogen  atom  occupies  the 
position  of  one  of  the  CHEE  groups  in  the  closed-chain. 


N  N 

Pyridine.  Quinoline. 

All  these,  and  many  other  compounds  and  their  derivatives, 
are  classed  as  aromatic,  because  they  show  the  general  be- 
haviour already  referred  to,  and  resemble  benzene  more  or  less 
closely  in  constitution. 


CHAPTER    XX. 

HOMOLOGUES    OF    BENZENE. 

Benzene,  the  simplest  hydrocarbon  of  the  aromatic  group, 
is  also  the  first  member  of  a  homologous  series  of  the  general 
formula  CTOH9n_6;  the  hydrocarbons  ,of  this  series  are  derived 
from  benzene  by  the  substitution  of  alkyl-gronps  for  hydrogen 
atoms,  just  as  the  homologous  series  of  paraffins  is  derived 
from  marsh-gas.  The  second  member,  toluene  or  methyl- 
benzene,  C6H5-CH3,  like  benzene  itself,  exists  in  only  one 
form,  but  the  next  higher  homologue,  which  has  the  mole- 
cular composition  C8H10,  occurs  in  four  isomeric  forms — 
namely,  as  ethylbenzene,  C6H5-C2H5,  and  as  ortho-,  meta-, 
and  para-dimethylbenzene,  C6H4(CH3)2 ;  on  passing  up  the 
series,  the  number  of  theoretically  possible  isomerides  rapidly 
increases. 


HOMOLOGUES  OP  BENZENE.  329 

By  substituting  a  methyl-group  for  one  atom  of  hydrogen  in 
the  hydrocarbon  C8H10,  for  example,  eight  isomerides  of  the  com- 
position C9H12  may  theoretically  be  obtained,  and  are,  in  fact, 
known  ;  of  these  isomerides,  five — namely,  propylbenzene  and  iso- 
propylbenzene,  C6H5-C3H7,  and  0-,  m-,  and  /?-methylethylbenzene, 
C6H4(CH3)-C2H5,  are  derived  from  ethylbenzene,  the  other  three 
— namely,  symmetrical,  adjacent,  and  asymmetrical  trimethyl- 
benzene,  C6H3(CH3)3,  being  derived  from  the  dimethylbenzenes. 

Most  of  the  hydrocarbons  of  this  series,  and  others  which 
will  be  mentioned  later,  occur  in  coal-tar,  from  which  they 
are  extracted  in  much  the  same  way  as  benzene ;  it  is, 
however,  exceedingly  difficult  to  obtain  any  of  them  in  a 
pure  state  directly  from  this  source  by  fractional  distillation, 
as  the  boiling-points  of  some  of  the  compounds  He  very  close 
together;  nevertheless,  the  process  is  now  carried  out  on 
the  large  scale  with  such  care  and  with  such  perfect  apparatus 
that  the  purified  compounds  contain,  in  some  cases,  only  traces 
of  foreign  substances. 

The  homologues  of  benzene  may  be  obtained  by  the 
following  general  methods : 

(1)  By  treating  benzene  or  its  homologues  with  alkyl 
halogen  compounds  in  presence  of  anhydrous  aluminium 
chloride  (Friedel  and  Craft's  reaction) ;  under  these  condi- 
tions the  hydrogen  atoms  of  the  nucleus  are  displaced  by 
alkyl-groups,  benzene  and  methyl  chloride,  for  example, 
giving  toluene,  C6H5-CH3,  xylene,  C6H4(CH3)2,  trimethyl- 
benzene,  C6H3(CH3)3,  &c4;  whereas  ethylbenzene,  with  the  same 
alkyl  compound,  yields  methylethylbenzene,  C6H4(CH3)-C2H5, 
dimethylethylbenzene,  C6H3(CH3)2-C2H5,  and  so  on.  These 
syntheses  may  be  expressed  by  equations  such  as  the 
following,  but  the  exact  nature  of  the  interaction  is  not 
known : 

C6H0  +  CH3C1  =  CfiH5-CH,  +  HC1 
C6H6  +  2CH3C1  -  C6H4(CH3)2  +  2HC1 
C6H5-C2H5  +  CH3C1  =  C6H4(CH8).C2H5  +  HC1. 

It    is    probable    that   an    aluminium    compound,    such   as 


330  HOMOLOGUES    OF    BENZENE. 

C6H5-A12C15,  is  first  formed  with  evolution  of  hydrogen 
chloride,  this  substance  then  interacting  with  the  alkyl  halogen 
compound  to  form  the  hydrocarbon,  aluminium,  chloride  being 
regenerated, 

C6H5-A12C15  +  CH3C1  =  C6H5.CH3  +  A12C16; 

an  alkyl  bromide  may  be  used  instead  of  the  chloride,  and 
anhydrous  ferric  or  zinc  chloride  may  be  employed  in  the 
place  of  aluminium  chloride,  but,  as  a  rule,  not  so  success- 
fully. 

Anhydrous  benzene,  or  one  of  its  homologues,  is  placed  in  a  flask 
connected  with  a  reflux  condenser,  and  about  one-third  of  its  weight 
of  anhydrous  aluminium  chloride  added ;  the  alkyl  chloride  or 
bromide  is  then  passed  into  the  liquid  if  a  gas,  or  poured  in,  if  a 
liquid,  and  the  mixture  heated  on  a  water-bath  until  the  evolution 
of  hydrogen  chloride  or  bromide  is  at  an  end ;  the  apparatus  and 
materials  must  be  dry.  In  some  cases  ether,  carbon  bisulphide,  or 
petroleum  is  previously  mixed  with  the  hydrocarbon  in  order  to 
dilute  it,  experience  having  shown  this  to  be  advantageous.  When 
quite  cold,  water  is  gradually  added  to  dissolve  the  aluminium 
compounds,  and  after  having  been  separated  and  dried  with  calcium 
chloride,  the  mixture  of  hydrocarbons  is  submitted  to  fractional 
distillation  ;  in  some  cases  a  preliminary  distillation  in  steam  is 
advisable.* 

(2)  By  treating  a  mixture,  consisting  of  a  halogen  deriva- 
tive of  benzene  or  of  one  of  its  homologues,  and  an  alkyl 
halogen  compound,  with  sodium  or  potassium  (Fittig's  re- 
action) ;  this  method  of  formation  is  similar  to  that  by  which 
the  higher  paraffins  may  iJti  HynllieticaHy  produced  from 
methane,  and  has  the  advantage  over  Friedel  and  Craft's 
method  that  the  constitution  of  the  product  is  known. 
Bromobenzene  and  methyl  iodide,  for  example,  give  toluene, 
whereas  o-,  m-,  or  j>bromotoluene  and  ethyl  iodide  yield  o-, 
m-,  or  ^-ethylmethylbenzene, 

C6H5Br  +  CH3I  +  2Na  =  C0H5.CH3  +  Nal  +  NaBr 
C6H4Br.CH3  4-  C2H5I  +  2K  =  C6H4<^  +  KBr  +  KI. 

*  In  most  cases  the  detailed  description  of  the  preparation  of  substances 
is  given  in  small  print* 


HOMOLOGUES    OF    BENZENE.  331 

The  bromo-derivatives  of  the  aromatic  hydrocarbons  are 
usually  employed  in  such  cases  because  the  chloro-derivatives 
are  not  so  readily  acted  on,  and  the  iodo-compounds  are  not 
so  easily  prepared ;  the  alkyl  iodides  are  also  used  in  pre- 
ference to  the  chlorides  or  bromides  because  they  interact  more 
readily. 

(3)  By   heating    carboxy-derivatives    of    benzene    and    its 
homologues    with    soda-lime,    a    method    analogous    to    that 
employed  in  converting  the  fatty  acids  into  paraffins, 

C«H4<COOH   =    C6H5'CH3    +    C°'2 

C6H4\CO  OH  =  CeHt5 

(4)  By  passing  the  vapour  of  hydroxy-derivatives  of  benz- 
ene and  its  homologues  over  heated  zinc-dust,  which  acts  as 
a  powerful  reducing  agent  by  combining  with  the  oxygen  in 
the  compound, 

CJEL.OH  +  Zn  =  (UL  +  ZriO 


C6H4<3  +  Zn  =  C6H5-CH3  +  ZnO. 

(5)  By  the  destructive  distillation  of  coal,  wood,  peat,  &c., 
and  by  passing  the  vapour  of  many  fatty  compounds  through 
red-hot  tubes  (compare  p.  300). 

General  Properties. — Most  of  the  homologues  of  benzene 
are  colourless,  mobile  liquids,  resembling  benzene  in  smell 
and  in  ordinary  physical  properties ;  one  or  two,  however, 
are  crystalline  at  ordinary  temperatures.  They  all  distil 
without  decomposition,  are  volatile  in  steam,  and  burn  with 
a  smoky  flame;  they  are  insoluble  in  water,  but  miscible 
with  alcohol,  ether,  petroleum,  &c.,  in  all  proportions ;  they 
dissolve  fats  and  many  other  substances  which  are  insoluble 
in  water. 

Just  as  in  other  homologous  series,  the  homologues  of 
benzene  show  a  gradual  variation  in  physical  properties  with 
increasing  molecular  weight ;  an  example  of  this  is  afforded 


332  HOMOLOGUES  OF  BENZENE. 

by  the  following  ?wo?i0-substitution  products  of  benzene,  only 
the  last  of  which  occurs  in  two  isomeric  forms : 

Benzene,  C6H6.    Toluene,  C7H8.   Ethylbenzene,  C8H10.  Fr°Pylbeuzene>  C9H12- 

Normal.          I  so. 

Sp.gr.  at  0°    0-899        0-882  0-866  (at  20°)         0-881        0-879 

B.p.  80-5°        110-3°  134°  157°         153°. 

In  the  case  of  the  cZi-substitution  products  the  gradual  variation 
in  physical  properties  is  obscured  by  the  existence  of  the 
three  (or  more)  isomeric  forms,  which  themselves  show 
considerable  differences,  as  illustrated  by  the  three  isomeric 
xylenes,  C6H4(CH3)2, 

Orthoxylene.  Metaxylene.  Paraxylene. 

Sp.  gr.  at  0°    0-893  0-881  0-880 

B.p.  142-143°  139°  136-137°  (M.p.  15°). 

As  a  general  rule,  to  which,  however,  there  are  some  ex- 
ceptions, para-compounds  melt  at  a  higher  temperature  than 
the  corresponding  meta-compounds,  and  the  latter  usually  at 
a  higher  temperature  than  the  corresponding  ortho-compounds; 
the  boiling-points  also  vary,  but  with  less  regularity. 

The  homologues  of  benzene  show  the  characteristic  chemical 
behaviour  of  the  simplest  hydrocarbon,  inasmuch  as  they 
readily  yield  nitro-  and  sulphonic-derivatives  j  toluene,  for 
example,  gives  nitrotoluene,  C6H4(CH3)-N02,  and  toluene- 
sulphonic  acid,  C6H4(CH3)-S03H,  xylene  yielding  nitro- 
xylene,  C6H3(CH3)2-N02,  and  xylenesulphonic  acid, 

C6H3(CH3)2.S03H.      (V 

In  these,  and  in  all  similar  reactions,  the  product  invariably 
consists  of  a  mixture  of  isomerides,  the  course  of  the  reaction 
depending  both  on  the  nature  of  the  interacting  compounds 
and  on  the  conditions  of  the  experiment  (compare  p.  351) ;  as 
a  rule,  the  greater  the  number  of  alkyl-groups  in  the  hydro- 
carbon, the  more  readily  it  yields  nitro-  and  sulphonic-deri- 
vatives. 

The  fact  that  benzene  and  its  homologues  gradually  dissolve 
in  concentrated  sulphuric  acid,  especially  on  warming,  is  some- 


I10MOLOGUES    OF    BENZENE.  333 

times  made  use  of  in  separating  these  aromatic  hydrocarbons 
from  the  paraffins,  as,  for  example,  in  the  analysis  of  coal- 
gas  ;  their  separation  from  unsaturated  fatty  hydrocarbons 
could  not  of  course  be  accomplished  in  this  way,  as  the  latter 
are  also  dissolved  by  concentrated  sulphuric  acid. 

All  the  homologues  of  benzene  are  very  stable,  and  are  with 
difficulty  resolved  into  compounds  containing  a  smaller  number 
of  carbon  atoms ;  powerful  oxidising  agents,  however,  such  as 
chromic  acid,  potassium  permanganate,  and  dilate  nitric  acid, 
act  on  them  slowly,  the  alkyl-groups  or  side-chains  being 
attacked,  and  as  a  rule  converted  into  carboxyl-groups ;  toluene 
and  ethylbenzene,  for  example,  give  benzoic  acid,  whereas  the 
xylenes  yield  dicarboxylic  acids  (p.  424), 

C6H5-CH3  +  30  =  C6H5.COOH  +  H20 
C6H5.CH2.CH3  +  60  =  C6H5.COOH  +  C02  +  2H20 
C6H4(CH3)2  +  60  -  C6H4(COOH)2  +  2H20. 

Although  in  most  cases  oxidation  leads  to  the  formation  of 
a  carboxy-derivative  of  benzene,  the  stable  nucleus  of  six 
carbon  atoms  remaining  unchanged,  some  of  the  homologues 
are  completely  oxidised  to  carbon  dioxide  (compare  p.  337), 
and  benzene  itself  undergoes  a  similar  change  on  prolonged 
and  vigorous  treatment. 

Aromatic  hydrocarbons,  like  those  of  the  fatty  series,  may 
be  regarded  as  hydrides  of  hypothetical  radicles;  in  other 
words,  radicles  may  theoretically  be  derived  from  aromatic 
hydrocarbons  by  taking  away  atoms  of  hydrogen.  These 
radicles  have  no  actual  existence,  but  the  assumption  is  useful 
in  naming  aromatic  compounds  ;  the  mono-  and  di-substitution 
products  of  benzene,  for  example,  may  be  regarded  as  com- 
pounds of  the  monovalent  radicle  phenyl,  Cy.H^-,  or  of  the 
divalent  radicle  phem/lme^. .  C6H4<C,  respectively,  as  in 
phenylamine  (aniline),  C6H5-NH2,  and  in  0-,  ra-  and  j9-phenyl- 
enediamine,  CgH^XH^ Toluene  derivatives,  again,  may 
be  named  as  if  they  were  derived  from  the  radicle  toluyl, 
CH3-C6H4-,  or  from  the  radicle  benzyl,  C6H5-CH2-,  according 


334  HOMOLOGUES  OF  BENZENE. 

as  hydrogen  of  the  nucleus,  or  of  the  side-chain,  has  been 
displaced.  The  compound  C6H5-CH2-OH,  for  example,  is 
called  benzyl  alcohol.  The  isomeric  hydroxy-compounds, 
C0H4(CH3)-OH,  however,  are  usually  known  as  the  (o.m.p.} 
cresols  (p.  396).  Other  hypothetical  radicles,  such  as  xylyl, 

f^TT 

C6H3(CH3)2-,  and  xylylene,  C6H4<^CH2~,  are  also  made  use 

of. 

Toluene,  methylbenzene,  or  phenylmethane,  C6H5-CH3, 
although  always  prepared  from  the  '90  per  cent,  benzol' 
separated  from  coal-tar  (p.  297),  can  be  obtained  by  any  of  the 
general  reactions  given  above,  and  also  by  the  dry  distillation 
of  balsam  of  Tolu  and  other  resins. 

The  commercial  substance  is  invariably  impure,  and  when 
shaken  with  concentrated  sulphuric  acid  it  colours  the  acid 
brown  or  black.  It  may  be  purified  by  repeated  fractional 
distillation,  but  even  then  it  will  contain  thiotolene,  CgHgSj^ 
homologue  of  thiophene  (p.  300),  and  will  show  the  indo- 
phenin  reaction  (with  isatin  and  concentrated  sulphuric  acid). 

Pure  toluene  is  most  conveniently  prepared  from  balsam  of 
Tolu.  or  by  distilling  pure  toluic  acid  with  lime, 

C6H4<£*5H  =  C6H5.CH3  +  C02. 

It  is  a  colourless,  mobile  liquid  of  sp.  gr.  0-882  at  0°,  and 
boils  at  110°;  it  does  not  solidify  even  at  -28°,  and  cannot, 
therefore,  like  benzene,  be  purified  by  freezing.  It  resembles 
benzene  very  closely  in  most  respects,  differing  from  it  princi- 
pally in  those  properties  which  are  due  to  the  presence  of  the 
methyl-group.  Its  behaviour  with  nitric  acid  and  with  sul- 
phuric acid,  for  example,  is  similar  to  that  of  benzene,  inasmuch 
as  it  yields  nitro-  and  sulphonic-derivatives ;  these  compounds, 
moreover,  exist  in  three  isomeric  (o.m.p.)  forms,  since  they 
are  di-substitution  products  of  benzene.  The  presence  of  the 
methyl-group,  on  the  other  hand,  causes  toluene  to  show  in 
some  respects  the  properties  of  a  paraffin.  The  hydrogen 
of  this  methyl-group  may  be  displaced  by  chlorine,  for 


HOMOLOGUES    OF    BENZENE.  335 

example,  and  the  latter  by  a  hydroxyl-  or  amido-group,  by 
methods  exactly  similar  to  those  employed  in  bringing  about 
similar  changes  in  fatty  compounds,  substances  such  as 
C6H?.CH2C1,  C6H5-CH2.OH,  and  C6H5.CH2-ISTH2  being 
obtained.  This  behaviour  was  of  course  to  be  expected,  since 
toluene  or  phenylmethane  is  a  mono-substitution  product  of 
marsh-gas  just  as  much  as  a  derivative  of  benzene. 

The  next  homologue  of  toluene — namely,  the  hydrocarbon  of 
the  molecular  formula  C8H10,  exists  in  the  following  four 
isomeric  forms,  of  which  the  three  yylenes  or  dimethylbenzenes 
are  the  most  important. 


CH3 

Orthoxylene.  Metaxylene.  Paraxylene.  Ethylbenzene. 

The  three  xylenes  occur  in  coal-tar,  and  may  be  partially 
separated  from  the  other  constituents  of  '  50  per  cent,  benzol ' 
(p.  297)  by  fractional  distillation.  The  portion  boiling  at 
136-141°,  after  repeated  distillation  contains  a  large  quantity 
(up  to  85  per  cent.)  of  w-xylene  and  smaller  quantities  of  the 
o-  and  ^-compounds ;  the  three  isomerides  cannot  be  separated 
from  one  another  or  from  all  impurities  by  further  distilla- 
tion, or  by  any  simple  means,  although  it  is  possible  to  obtain 
a  complete  separation  by  taking  advantage  of  differences  in 
chemical  behaviour. 

??i-Xylene  is  readily  separated  from  the  other  isomerides  by  digest- 
ing with  dilute  nitric  acid,  which  oxidises  o-  and  jo-xylene  to  the 
corresponding  tolnic  acids,  CfiH4(CH3)-COOH,  but  does  not  attack  m- 
xylene;  the  product  is  rendered  alkaline  by  the  addition  of  potash, 
and  the  unchanged  hydrocarbon  purified  by  distillation  in  steam  and 
fractionation.  The  isolation  of  o-  and  />-xylene  depends  on  the  follow- 
ing facts  :  (1)  When  crude  xylene  is  agitated  with  concentrated 
sulphuric  acid,  o-  and  m-xylene  are  converted  into  sulphonic 
acids,  C0H,(CH3).,-SO3H ;  />xylene  remains  unchanged,  as  it  is 


OOO  HOMOLOGUES    OF    BENZENE. 

only  acted  on  by  fuming  sulphuric  acid.  (2)  The  sodium  salt  of 
o-xylenesul  phonic  acid  is  less  soluble  in  water  than  the  sodium 
salt  of  m-xylenesulphonic  acid  ;  it  is  purified  by  recrystallisation, 
and  converted  into  o-xylene  by  heating  with  hydrochloric  acid 
under  pressure  (p.  381). 

The  three  xylenes  may  all  be  prepared  by  one  or  other  of 
the  general  methods :  when,  for  example,  methyl  chloride  is 
passed  into  benzene  in  presence  of  aluminium  chloride, 
o-xylene  and  a  small  quantity  of  the  j?-compound  are  obtained, 

C6H6  +  2CH3C1  =  C6H4(CH3)2  +  2HC1; 

toluene,  under  the  same  conditions,  yields  the  same  two 
compounds, 

C6H5-CH3  +  CH3C1  =  C6H4(CH3)2  +  HC1. 
The    non-formation    of    ??i-xylene    in    these    two    cases    is 
accounted  for  by  assuming  that  the  methyl-group  first  intro- 
duced   into   the  benzene  molecule  exerts  some  directing  in- 
fluence on  the  position  taken  up  by  the  second  one  (p.  351). 

Orthoxylene  is  obtained  in  a  state  of  purity  by  treating 
o-bromotoluene  with  methyl  iodide  and  sodium, 

CfiH4<B? 3  +  CH3I  +  2Na  *  C6H4<™3  +  NaBr  +  Nal, 

3 

pure  paraxylene  being  produced  in  a  similar  manner  from  p- 
bromotoluene ;  metaxylene  cannot  be  prepared  by  treating 
ra-bromotoluene  with  methyl  iodide  and  sodium,  but  is  easily 
obtained  in  a  pure  condition  by  distilling  mesitylenic  acid 
(p.  338)  with  lime, 

C6HS(CHS)2.COOH  =  C6H4(CH3)2  +  C02. 
The  three  xylenes  are  very  similar  in  physical  properties 
(compare  p.  332),  being  all  colourless,  mobile,  rather  pleasant- 
smelling,  inflammable  liquids  (p-xylene  melts  at  15°),  which 
distil  without  decomposition,  and  are  readily  volatile  in  steam. 
They  also  resemble  one  another  in  chemical  properties,  although 
in  some  respects  they  show  important  differences,  which  must 
be  ascribed  to  their  difference  in  constitution.  On  oxidation, 
under  suitable  conditions,  they  are  all  converted  in  the  first 


HOMOLOGUES  OF  BENZENE.  337 

place  into  monocarboxylic  acids  which  are  represented  by  the 
formulae 


-COOH 

-  -COOH 


COOH 

Orthotoluic  Acid.  Metatoluic  Acid.  Paratoluic  Acid 

On  further  oxidation  the  second  methyl-group  undergoes  a 
like  change,  and  the  three  corresponding  dicarboxylic  acids, 
C6H4(COOH)2,  are  formed  (p.  424). 

The  three  hydrocarbons  show,  however,  slight  differences  in 
behaviour  on  oxidation,  one  being  more  easily  acted  on  than 
another  by  a  particular  oxidising  agent.  With  chromic  acid,  for 
example,  o-xylene  is  completely  oxidised  to  carbon  dioxide,  whereas 
wi-xylene  and  ^?-xylene  yield  the  dicarboxylic  acids  (see  above) ; 
with  dilute  nitric  acid  o-xylene  gives  o-toluic  acid,  and  ^?-xylene 
jo-toluic  acid,  but  m-xylene  is  not  acted  on.  Their  behaviour  with 
sulphuric  acid  is  also  different  (p.  335). 

Ethylbenzene,  or  phenyletliane,  C6H5-C2H5,  an  isomeride  of 
the  xylenes,  is  not  of  much  importance ;  it  occurs  in  coal-tar, 
and  may  be  obtained  by  the  general  methods.  It  is  a  colour- 
less liquid,  boiling  at  134°,  and  on  oxidation  with  dilute  nitric 
acid  or  chromic  acid  it  is  converted  into  benzoic  acid, 

C6H5.CH2.CH8  +  60  -  C6H5-COOH  +  C02  +  2H20. 

The  next  member  of  the  series  has  the  molecular  formula 
C9H12,  and  exists,  as  already  pointed  out  (p.  329),  in  eight 
isomeric  forms,  of  whicK  the  three  trimethylbenzenes  and 
isopropylbenzene  are  the  most  important. 

Mesitylene,  or  symmetrical  irimetliylbenzene, 

CH3 


occurs  in  small  quantities  in  coal-tar,  but  is  most  conveniently 

V 


338  HOMOLOGUES    OF    BENZENE. 

'prepared  by  distilling  a  mixture  of  acetone  (2  vols.),  concen- 
/  trated  sulphuric  acid  (2  vols.),  and  water  (1  vol.),  sand  being 
-,  added  to  prevent  frothing, 

\.  3(CH3)2CO  =  C6H3(CH3)3  +  3H20. 

The  formation  of  mesitylene  in  this  way  is  not  only  of  interest 
•  because  it  affords  a  means  of  synthesising  the  hydrocarbon  from  its 
elements,  but  also  because  it  throws  light  on  the  constitution  of  the 
compound.  Although  the  change  is  a  process  of  condensation,  and 
is  most  simply  expressed  by  the  graphic  equation  already  given 
(p.  324),  it  might  be  assumed  that  the  acetone  is  first  converted 
into  CH3-C  iCH,  or  into  CH3-C(OH):CH2  (by  intramolecular  change), 
and  that  mesitylene  is  then  produced  by  a  secondary  reaction  ; 
whatever  view,  however,  is  adopted  as  to  the  actual  course  of  the 
reaction  (unless,  indeed,  highly  improbable  assumptions  be  made), 
the  final  result  is  always  the  same,  and  the  constitution  of  the 
product  must  be  expressed  by  a  symmetrical  formula ;  for  this, 
and  other  reasons,  mesitylene  is  regarded  as  symmetrical  or  1:3:5- 
trimethylbenzene. 

Mesitylene  is  a  colourless,  mobile,  pleasant-smelling  liquid, 
boiling  at  163°,  and  volatile  in  steam;  when  treated  with 
concentrated  nitric  acid  it  yields  mono-  and  di-nitromesitylene, 
whereas  with  a  mixture  of  nitric  and  sulphuric  acids  it  is 
converted  into  trinitromesitylene,  C6(N02)3(CH3)3.  On 
oxidation  with  dilute  nitric  acid  it  yields  mesitylenic  acid, 
C6H3(CH3)2.COOH,  uvitic  acid,  C6H3(CH3)(COOH)2,  and 
tritnesic  acid,  C6H3(COOH)3,  by  the  successive  transformation 
of  the  methyl-  into  carboxyl-groups. 

Pseudocitmene,  or  asymmetrical  trimethylbenzene,  C6H3(CH3)3 
[3CH3  =  1:2:4],  and  hemimeUitcnc,  or  adjacent  trimethylbenzene 
[3CH3  =  1:2:3],  also  occur  in  small  quantities  in  coal-tar,  and 
are  very  similar  to  mesitylene  in  properties;  on  oxidation,  they 
yield  various  acids  by  the  conversion  of  one  or  more  methyl-  into 
carboxyl-groups. 

Cumene,  or  isopropylbenzene,  CfiHfi-CH(CHa)9,  is  usually 
obtained  from  coal-tar ;  it  may  be  prepared  in  a  pure  condi- 
tion by  distilling  cumic  acid  (isopropylbenzoic  acid)  with  lime, 

=  C6H6'C3H7  +  C02, 


HOMOLOGUES    OF    BENZENE.  339 

by  treating  a  mixture  of  isopropyl  bromide  and  benzene  with 
aluminium  chloride, 

C6H6  +  C3H7Br  =  C6H5.C3H.  +  HBr, 

and  by  the  action  of  sodium  on  a  mixture  of  broniobenzene 
and  isopropyl  bromide, 

C6H5Br  +  C3H7Br  +  2Xa  =  C6H5.C3H7  +  2XaBr. 

It  is  a  colourless  liquid,  boiling  at  153°,  and  on  oxidation  with 
dilute  nitric  acid  it  is  converted  into  benzoic  acid. 

Cymene,  or  ^am-methylisopropylbenzene,  C6H4(Ciy -Cyi*, 
is  a  hydrocarbon  of  considerable  importance,  as  it  occurs  in  tlie 
ethereal  oils  or  essences  of  many  plants ;  it  is  easily  prepared 
in  many  ways,  as,  for  example,  by  heating  camphor  with 
phosphorus  pentoxide  or  phosphorus  pentasulphide, 

CioHi6°  =  C10H14  +  H20, 

by  heating  turpentine  with  concentrated  sulphuric  acid  or 
with  iodine  (both  of  which,  in  this  case,  act  as  oxidising 
agents), 

CioH16  +  0  =  C10H14  +  H20, 

and  by  heating  thymol  (p.  397),  or  carvacrol  (p.  397),  with 
phosphorus  pentasulphide  (which  acts  as  a  reducing  agent), 

!'    +  2H  =  C^HX^3   +  H00. 


Cymene  is  a  pleasant-smelling  liquid  of  sp.  gr.  0*8722  at  0°, 
and  boils  at  175-176°;  on  oxidation  with  dilute  nitric  acid 
it  yields  p-toluic  acid,  C6H4(CH3)-COOH,  and  terephthahc 
acid,  C6H4(COOH)2. 

Diphenyl,  Diphenylmetliane,  and  Triplienylmethane. 

All  the  hydrocarbons  hitherto  described  contain  only  one 
benzene  nucleus,  and  may  be  regarded  as  derived  from 
benzene  by  the  substitution  of  fatty  alkyl-groups  for  atoms 
of  hydrogen ;  there  are,  however,  several  other  series  of 
aromatic  hydrocarbons,  which  include  compounds  of  very 
considerable  importance. 


340  DIPHENYL,  DIPHENYLMETHANE. 

Diphenyl,  C6H5-C6H5,  contains  two  benzene  nuclei,  and 
is  the  hydrocarbon  in  the  aromatic  series  which  corresponds 
with  dimethyl  in  the  fatty  series,  although  it  is  not  a  homo- 
logue  of  benzene.  It  is  formed  on  treating  bromobenzene  in 
ethereal  solution  with  sodium, 

2C6H5Br  +  2Na  =  (£H5.C6H5  +  2NaBr, 

the  reaction  being  analogous  to  the  formation  of  dimethyl 
from  methyl  bromide  by  the  action  of  sodium. 

Diphenyl  is  prepared  by  passing  benzene  vapour  through  a  red- 
hot  tube  filled  with  pieces  of  pumice, 

2C6H6  =  C6H5.C6H5  +  H2. 

The  dark-coloured  distillate  is  fractionated,  and  the  diphenyl  puri- 
fied by  recrystallisation  from  alcohol. 

Diphenyl  is  a  colourless,  crystalline  substance,  melts  at  71°, 
and  boils  at  254° ;  when  oxidised  with  chromic  acid,  it  yields 
benzoic  acid,  one  of  the  benzene  nuclei  being  destroyed. 
Its  behaviour  with  halogens,  nitric  acid,  and  sulphuric  acid  is 
similar  to  that  of  benzene,  substitution  products  being  formed. 

Diphenylmethane,  C6H5-CH2-C6H5,  also  contains  two  ben- 
zene nuclei ;  it  may  be  regarded  as  derived  from  marsh-gas 
by  the  substitution  of  two  phenyl-groups  for  two  atoms  of 
hydrogen,  just  as  toluene  or  phenylmethane  may  be  considered 
as  a  mono-substitution  product  of  methane. 

Diphenylmethane  may  be  prepared  by  treating  benzene 
with  benzyl  chloride  (p.  348)  in  presence  of  aluminium 
chloride, 

C6H6  +  C6H5.CH2C1  =  C6H5.CH2-C6H5  +  HC1. 

It  is  a  crystalline  substance,  and  melts  at  26-5°;  when 
treated  with  nitric  acid,  it  yields  nitro-derivatives  in  the 
usual  way,  and  on  oxidation  with  chromic  acid,  it  is  con- 
verted into  diphenyl  ketone  or  benzophenone,  C6H5-CO-C6H5 
(p.  412). 


Triphenylmethane,  (C6H5)3CH,  is  by  far  the  most  im- 
portant member  of  another  series,  the  members  of  which 
contain  three  benzene  nuclei.  It  is  formed  when  benzal 


TRIPHENYLMETHANE.  341 

chloride  (p.  349)  is  treated  with  benzene  in  presence  of 
aluminium  chloride, 

C6H5.CHC12  +  2C6H6  =  (C6H5)3CH  +  2HC1, 
but  it  is  usually  prepared  by  heating  a  mixture  of  chloroform 
and  benzene  with  aluminium  chloride, 

CHC13  +  3C6H6  =  (C6H5)3CH  +  3HC1. 

Aluminium  chloride  (5  parts)  is  gradually  added  to  a  Imxlure 
of  chloroform  (1  part)  and  benzene  (5  parts),  which  is  then  heated 
at  about  60°  until  the  evolution  of  hydrogen  chloride  ceases,  an 
operation  occupying  about  thirty  hours ;  after  cooling  and  adding 
water,  the  oily  product  is  separated  and  submitted  to  fractional 
distillation ;  those  portions  of  the  distillate  which  solidify  on 
cooling,  consist  of  crude  triphenylmethane,  which  is  further  purified 
by  recrystallisation  from  benzene  and  then  from  ether. 

Triphenylmethane  is  a  colourless,  crystalline  compound, 
which  melts  at  93°,  and  boils  at  355° ;  it  is  readily  soluble 
in  ether  and  benzene,  but  only  sparingly  so  in  cold  alcohol. 
"When  treated  with  fuming  nitric  acid,  it  is  converted  into 
a  yellow,  crystalline  tfn'mYro-derivative,  CH(C6H4-N02)3, 
which,  like  other  nitro-compounds,  is  readily  transformed 
into  the  corresponding  triamido-compo\md, 
CH(C6H4-NH2)3, 

on  reduction ;  the  last-named  substance  is  of  considerable 
importance,  as  many  of  its  derivatives  are  largely  employed 
as  dyes  (p.  508). 

On  oxidation  with  chromic  acid,  triphenylmethane  is  con- 
verted into  triphenyl  carbinol,  (C6H5)3C-OH. 


CHAPTER     XXL 

HALOGEN    DERIVATIVES    OF    BENZENE    AND    ITS    HOMOLOGUES. 

The  action  of  halogens  on  benzene  has  already  been  referred 
to  (p.  302),  and  it  has  been  pointed  out  that  the  hydrocarbon 
yields  either  additive  or  substitution  products  according  to 


342  HALOGEN    DERIVATIVES    OF    BENZENE,    ETC. 

the  conditions  of  the  experiment ;  at  ordinary  temperatures, 
in  absence  of  direct  sunlight,  substitution  products  are 
formed,  the  action  being  greatly  hastened  by  the  presence 
of  a  halogen  carrier,  such  as  iodine,  ferric  chloride,  or 
antimony  chloride;*  at  its  boiling-point,  however,  or  in 
presence  of  direct  sunlight,  the  hydrocarbon  yields  additive 
compounds  by  direct  combination  with  (two,  four,  or)  six  • 
atoms  of  the  halogen. 

The  homologues  of  benzene  also  show  a  curious  behaviour ; 
when  treated  with  chlorine  or  bromine  at  ordinary  tempera- 
tures in  absence  of  direct  sunlight,  they  are  converted  into 
substitution  products  by  the  displacement  of  hydrogen  of  the 
nucleus,  and,  as  in  the  case  of  benzene  itself,  interaction  is 
greatly  promoted  by  the  presence  of  a  halogen  carrier ;  under 
these  conditions  toluene,  for  example,  gives  a  mixture  of  o- 
and  p-chlorotoluenes  or  bromotoluenes, 

C6H5-CH3  +  C12  -  C6H4<{?H  +  HCL 

3 

When,  on  the  other  hand,  no  halogen  carrier  is  present, 
and  the  hydrocarbons  are  treated  with  chlorine  or  bromine 
at  their  boiling-points,  or  in  direct  sunlight,  they  yield  de- 
rivatives by  the  substitution  of  hyjdrogen  of  the  side-chain; 
when,  for  example,  chlorine  is  passed  into  boiling  toluene, 
the  three  hydrogen  atoms  of  the  methyl-group  arePsucces- 
sively  displaced,  benzyl  chloride,  C6H5-CH2C1,  benzal  chloride, 
C6H5'CHC19,  and  benzotrichloride,  C6H5-CC13,  being  formed; 
xylene,  again,  when  treated  with  bromine  at  its  boiling-point, 
gives  the  compounds 

pTT</CH2Br         ,    PTT  /CH2Br 
C6H4<;CH2        and    C6H4<CH2Br. 

*  The  action  of  iodine  has  been  explained  in  part  i.  (p.  163) ;  ferric 
chloride,  antimony  pentachloride,  molybdenum  pentachloride,  and  other 
metallic  chlorides,  act  as  halogen  carriers,  probably  because  they  readily 
dissociate,  yielding  nascent  halogen  and  lower  chlorides  (FeCl2,  SbCl3, 
MoCl3) ;  the  latter  then  combine  again  with  a  fresh  quantity  of  the  halogen, 
and  thus  the  process  is  repeated. 


HALOGEN    DERIVATIVES    OF   BENZENE,    ETC.  343 

Although  these  statements  are  true  in  the  main,  it  must 
not  be  supposed  that  substitution  takes  place  exclusively 
either  in  the  nucleus  or  side-chain,  as  the  case  may  be,  be- 
cause this  is  not  so ;  in  presence  of  a  halogen  carrier  traces 
of  a  halogen  derivative  are  formed  by  substitution  of  hydro- 
gen of  the  side-chain,  and  at  the  boiling-point  of  the  hydro- 
carbon, or  in  direct  sunlight,  traces  of  a  substitution  product, 
formed  by  displacement  of  hydrogen  of  the  nucleus,  are 
obtained. 

Iodine  seldom  acts  on  benzene  and  its  homologues  under 
any  of  the  above-mentioned  conditions,  partly  because  of  the 
slight  affinity  of  iodine  for  hydrogen,  partly  because  the 
hydrogen  iodide  which  is  produced  interacts  with  the  iodo- 
derivative,  and  reconverts  it  into  the  hydrocarbon 

C6H6  +  I,  =  C6H5I  +  HI 
C6H5I  +  HI  =  C6H6  +  I2; 

if,  however,  iodic  acid,  or  some  other  substance  which 
decomposes  hydriodic  acid,  be  present,  iodo-derivatives 
may  sometimes  be  prepared  by  direct  treatment  with  the 
halogen.* 

Preparation. — As  a  rule,  chloro-  and  bromo-derivatives  of 
benzene  and  its  homologues  are  prepared  by  direct  '  chlorina- 
tion '  or  '  bramination,1  the  conditions  employed  depending  on 
whether  hydrogen  of  the  nucleus  or  of  the  side-chain  is  to 
be  displaced ;  if,  for  example,  it  were  desired  to  convert 
toluene  into  p-chlorobenzyl  chloride,  C6H4C1-CH2C1,  the 
hydrocarbon  might  be  first  treated  with  chlorine  at  ordinary 
temperatures  in  presence  of  iodine,  and  the  p-chlorotoluene, 
C6H4C1-CH3,  after  having  been  separated  from  the  accom- 
panying ortho-compound,  would  then  be  heated  to  boiling  in 

*  HIO3  +  5HI  =  3I2  +  3H2O.  lodo-substitution  products  are  also  fre- 
quently formed  on  employing  FeC^,  or  AlC^,  as  a  carrier,  because  the 
IC1  which  is  formed  has  a  much  more  energetic  substituting  action  than 
the  iodine  itself,  owing  to  the  simultaneous  formation  of  HC1, 

C6H6  +  IC1  =  C6H5I  +  HC1. 


344  HALOGEN    DERIVATIVES    OF   BENZENE,    ETC. 

a  flask  connected  with  a  reflux  condenser,  and  a  stream  of 
dry  chlorine  led  into  it. 

In  all  operations  of  this  kind  the  theoretical  quantity,  or  a  slight 
excess  of  halogen,  is  employed  ;  the  bromine  is  weighed  directly, 
but  the  weight  of  the  chlorine  is  usually  ascertained  indirectly  by 
continuing  the  process  until  the  theoretical  gain  in  weight  has 
taken  place  ;  the  halogen  should  be  dry,  as  in  presence  of 
water  oxidation  products  of  the  hydrocarbon  may  be  formed. 
The  fumes  of  hydrogen  chloride  or  bromide  evolved  during  such 
operations  are  conveniently  absorbed  by  passing  them  to  the 
bottom  of  a  deep  vessel  containing  damp  coke. 

A  very  important  general  method  for  the  preparation  of 
aromatic  halogen  derivatives,  containing  the  halogen  in  the 
nucleus,  consists  in  the  decomposition  of  the  diazo-compounds. 
As  the  properties  and  decompositions  of  the  last-named 
substances  are  described  later  (p.  370),  it  is  only  necessary 
to  state  here  that  this  method  is  used  in  the  preparation 
of  nearly  all  iodo-compounds,  and  that  it  affords  a  means 
of  indirectly  substituting  any  of  the  halogens,  not  only  for 
hydrogen,  but  also  for  nitro-  or  amido-groups. 

The  conversion  of  benzene  or  toluene,  for  example,  into  a 
mono-halogen  derivative  by  this  method  involves  the  follow- 
ing steps  : 

C6H6      C6H5.M)2      C6H5-NH2     C6H5.N:NC1      C6H5C1 

Benzene.      Nitrobenzene.      Amidobenzene.          Diazobenzene       Chlorobenzene. 

Chloride. 

C6H5.CH3       C6H4<N08       C6H4<NHs      C6H4<N: 

Toluene.  Nitrotoluene.  Amidotoluene.  Diazotoluene 

Bromide. 


Bromotoluene. 


The  preparation  of  a  eft-halogen  derivative  may  sometimes 
be  carried  out  in  a  similar  manner,  the  hydrocarbon  being 
first  converted  into  the  eft-nitro-derivative  ;  in  most  cases, 
however,  it  is  necessary  to  prepare  the  ??iowo-halogen  derivative 
by  the  reactions  given  above,  and  after  converting  it  into 


HALOGEN    DERIVATIVES    OF    BENZENE,    ETC.  345 

the  nitro-compound,  the  nitro-group  is  displaced  by  a  second 
atom  of  halogen  by  repeating  the  series  of  operations. 

f1  H  T5r         C  H  xx^r          p  TT  x^^r  n  TJ  /''Br 

Bromobenzene.          Nitrobromo-  Amidobromo-  Diazobromo- 

benzene.  benzene.  benzene  Chloride. 

CH<Br 

Bromochlorobenzene. 

Halogen  derivatives  of  benzene  and  its  homologues  are  some- 
times prepared  by  treating  hydroxy -compounds  with  pentachloride 
or  pentabromide  of  phosphorus,  the  changes  being  similar  to  those 
which  occur  in  the  case  of  fatty  hydroxy-compounds ;  if  the 
hydroxyl-group  be  present  in  the  nucleus,  the  halogen  naturally 
takes  up  the  same  position,  phenol,  for  example,  giving  chloro- 
benzene,  and  cresol,  chlorotoluene, 

C6H5-OH  +  PC15  =  C6H5C1  +  POC13  +  HC1 

CTT      -XL>.tl.3      i       "DPI       O    XJ   X^^-"-3  _l_    "POP1        _L    WPl    . 
gJtl4<-^/~vTq      T    i  v-'ig  —    I^g£l4<sx^-ii         T    JTVJ1^13   T    n^J  , 

an  aromatic  alcohol  (p.  402),  such  as  benzyl  alcohol,  also  yields  the 
corresponding  halogen  derivative  (benzyl  chloride),  containing  the 
halogen  in  the  side-chain, 


Halogen  derivatives  may  also  be  obtained  by  distilling  halogen 
acids  with  lime, 

C6H4<g?OH  =  C6H5Br  +  C02, 

by  heating  sulphonic  chlorides  (p.  381)  with  phosphorus  penta- 
chloride, 

C6H5.S02C1  +  PC15  =  C6H5C1  +  POC13  +  SOC12, 

and  by  several  other  methods  of  less  importance. 

Properties. — At  ordinary  temperatures,  some  of  the  halogen 
derivatives  of  benzene  and  its  homologues  are  colourless 
liquids ;  the  majority,  however,  are  crystalline  solids.  They 
are  all  insoluble,  or  nearly  so,  in  water,  but  readily  soluble 
in  alcohol,  ether,  &c.  Many  are  readily  volatile  in  steam, 
and  distil  without  decomposition,  the  boiling-point  being 
higher  and  the  specific  gravity  greater  than  that  of  the  parent 


346  HALOGEN    DERIVATIVES    OF    BENZENE,    ETC. 

hydrocarbon,    and   rising   also   on   substituting    bromine   for 
chlorine,  or  iodine  for  bromine. 

Benzene.        Chlorobenzene.    Bromobenzene.    lodobenzene. 

B.p 80-5°  132°  155°  185° 

Sp.gr.  at  0°         0-899  1428  1-521  1-857. 

They  are  not  so  inflammable  as  the  hydrocarbons,  and  the 
vapours  of  many  of  them  have  a  very  irritating  action  on  the 
eyes  and  respiratory  organs. 

When  the  halogen  is  united  with  carbon  of  the  benzene 
nucleus,  it  is,  as  a  rule,  very  firmly  combined,  and  cannot, 
as  in  the  case  of  the  halogen  derivatives  of  the  fatty  series, 
be  displaced  by  the  hydroxyl-  or  amido-group  with  the  aid  of 
aqueous  potash  or  ammonia  j  such  halogen  derivatives,  more- 
over, are  not  acted  on  by  alcoholic  potash,  and  cannot  be 
converted  into  less  saturated  compounds  in  the  same  way  as 
ethyl  bromide,  for  example,  may  be  converted  into  ethylene ; 
in  fact,  no  derivative  of  benzene,  containing  less  than  six 
monovalent  atoms,  or  their  valency  equivalent,  is  known. 
If,  however,  hydrogen  of  the  nucleus  has  been  displaced  by 
one  or  more  nitro-groups,  as  well  as  by  a  halogen,  the  latter 
often  becomes  much  more  open  to  attack ;  o-  and  j9-chloro- 
nitrobenzene,  C6H4C1-N02,  for  example,  are  moderately  easily 
acted  on  by  alcoholic  potash  and  by  alcoholic  ammonia  at 
high  temperatures,  yielding  the  corresponding  nitrophenols, 
C6H4(OH)-N02,  and  nitranilines,  C6H4(NH2).N02  ;  '  m- 
chloronitrobenzene,  however,  is  not  acted  on  under  these 
conditions,  a  fact  which  shows  that  compounds  closely  related 
in  constitution  and  identical  in  composition  sometimes  differ 
very  considerably  in  properties. 

Halogen  atoms  in  the  side-chains  are  very  much  less  firmly 
combined  than  those  in  the  nucleus,  and  may  be  displaced  by 
hydroxyl-  or  amido-groups  just  as  in  fatty  compounds ;  benzyl 
chloride,  C6H5-CH9C1,  for  example,  is  converted  into  benzyl 
alcohol,  C6H5-CH2-OH,  by  boiling  sodium  carbonate  solution, 
and  when  heated  with  alcoholic  ammonia  it  yields  benzyl- 
amine,  C6H5-CH2-NH2  (p.  368). 


HALOGEN    DERIVATIVES    OF    BENZENE,    ETC.  347 

Halogen  atoms  in  the  nucleus,  as  well  as  those  in  the  side- 
chain,  are  displaced  by  hydrogen  on  treatment  with  hydriodic 
acid  and  amorphous  phosphorus  at  high  temperatures,  or  with 
sodium  amalgam  in  alcoholic  solution ;  the  former,  however, 
are  much  less  readily  displaced  than  the  latter. 

Chlorobenzene,  or  phenyl  chloride,  C6H5C1,  may  be  de- 
scribed as  a  typical  example  of  those  halogen  derivatives  in 
which  the  halogen  is  combined  with  carbon  of  the  benzene 
nucleus.  It  may  be  obtained  (together  with  dichlorobenzene, 
C6H4C12,  trichlorobenzene,  C6H3C13,  &c.)  by  chlorinating 
benzene;  also  by  treating  phenol  (p.  391)  with  phosphorus 
pentachloride,  just  as  ethyl  chloride  may  be  produced  from 
alcohol, 

C6H5-OH  +  PC15  =  C6H5C1  +  POC13  +  HC1. 
It  is  usually  prepared  by  Sandmeyer's  reaction  (p.  372) — that 
is  to  say,  by  warming  an  aqueous  solution  of  diazobenzene 
chloride  with  cuprous  chloride ;  this  method,  therefore,  affords 
a  means  of  preparing  chlorobenzene,  not  only  from  the  diazo- 
compound,  but  also  indirectly  from  amidobenzene  (aniline), 
nitrobenzene,  and  benzene,  the  changes  being  those  given 
above  (p.  344).  Chlorobenzene  is  a  colourless,  mobile,  pleasant- 
smelling  liquid,  specifically  heavier  than  water;  it  boils  at 
132°,  and  is  readily  volatile  in  steam.  Like  benzene,  it  is 
capable  of  yielding  nitro-,  amido-,  and  other  derivatives  by 
the  displacement  of  one  or  more  hydrogen  atoms ;  it  differs 
from  ethyl  chloride  and  from  other  fatty  alkyl  halogen  com- 
pounds in  being  unacted  on  by  water  and  alkalies,  or  by 
metallic  salts ;  it  is  impossible,  for  example,  to  prepare  phenyl 
acetate,  CH3-COOC6H5,  by  treating  silver  acetate  with  chloro- 
benzene, although  ethyl  acetate  is  easily  obtained  from  ethyl 
chloride  in  this  way. 

Bromobenzene,  or  phenyl  bromide,  C6H5Br,  may  be  ob- 
tained by  bromiriating  benzene,  but  is  usually  prepared  from 
diazobenzene  bromide  by  Sandmeyer's  method  ;  it  is  a  colour- 
less liquid,  boiling  at  155°,  and  closely  resembles  chlorobenz- 
ene in  all  respects.  As  a  rule,  however,  the  bromo-deriva- 


348  HALOGEN   DERIVATIVES   OF   BENZENE,    ETC. 

tives  crystallise  more  readily,  and  have  a  higher  melting-point 
than  the  corresponding  chloro-compounds. 

lodobenzene,  or  phenyl  iodide,  boils  at  185°. 

Chlorotoluene,  or  toluyl  chloride,  C6H4C1-CH3,  being  a 
di-substitution  product  of  benzene,  exists  in  three  isomeric 
modifications,  only  two  of  which  —  namely,  the  o-  and  ^-com- 
pounds, are  formed  on  treating  cold  toluene  with  chlorine  in 
presence  of  iodine  or  ferric  chloride  ;  the  three  isomerides 
may  be  separately  prepared  by  treating  the  corresponding 
cresols  (p.  396)  with  phosphorus  pentachloride, 

+HC1> 


but  they  are  best  prepared  from  the  corresponding  toluidines 
by  Sandmeyer's  method, 


Toluidine.  Diazotoluene  Chloride.  Chlorotoluene. 

Orthoclilorotoluene  boils  at  156°,  metaclilorotoluene  at  150°, 
and  parachlorotoluene  at  160°;  they  resemble  chlorobenzene  in 
most  respects,  but,  since  they  contain  a  methyl-group,  they 
have  also  some  of  the  properties  of  fatty  compounds;  on 
oxidation,  they  are  converted  into  the  corresponding  chloro- 
benzoic  acids,  C6H4CLCOOH,  just  as  toluene  is  transformed 
into  benzoic  acid. 

Benzyl  chloride,  C6H5-CH2C1,  although  isomeric  with  the 
three  chlorotoluenes,  differs  from  them  very  widely,  and  may 
be  taken  as  an  example  of  the  class  of  halogen-compounds 
in  which  the  halogen  is  present  in  the  side-chain.  It  can 
be  obtained  by  treating  benzyl  alcohol  (p.  403)  with  phos- 
phorus pentachloride, 

C6H5.CH2.OH  +  PC15  -  C6H5.CH2C1  +  POC13  +  HC1, 

but   is    always    prepared   by   passing   chlorine   into   boiling 
toluene, 

C6H5.CHS  +  C12  =  C6H5-CH2C1  +  HC1. 
The  toluene  is  contained  in  a  tiask  which  is  heated  on  a  sand- 


HALOGEN  DERIVATIVES  OF  BENZENE,  ETC.       349 

bath  and  connected  with  a  reflux  condenser ;  a  stream  of  dry 
chlorine  is  then  passed  into  the  boiling  liquid  until  the  theoretical 
gain  in  weight  has  taken  place  and  the  product  is  purified  by 
fractional  distillation;  the  action  takes  place  most  rapidly  in 
strong  sunlight. 

Benzyl  chloride  is  a  colourless,  unpleasant-smelling  liquid, 
boiling  at  176°;  it  is  insoluble  in  water,  but  miscible  with 
alcohol,  ether,  benzene,  &c.  It  behaves  like  other  aromatic 
compounds  towards  nitric  acid,  by  which  it  is  converted  into 
a  mixture  of  isomeric  nitro-compounds,  C6H4(X02)-CH2C1. 
At  the  same  time,  however,  it  has  many  properties  in  com- 
mon with  the  alkyl  halogen  compounds ;  like  ethyl  chloride, 
it  is  slowly  decomposed  by  boiling  water,  yielding  the  cor- 
responding hydroxy-compound,  benzyl  alcohol  (p.  403), 

C6H5.CH2C1  +  H20  =  C6H5.CH2.QH  +  HC1, 

and  just  as  ethyl  chloride  interacts  with  silver  acetate,  giving 
ethyl  acetate,  so  benzyl  chloride,  under  the  same  conditions, 
yields  the  ethereal  salt,  benzyl  acetate, 

C6H5-CH2C1  +  CH3.COOAg  -  CH3.COOCH2-C6H5  +  AgCl. 

Benzyl  chloride  is  a  substance  of  considerable  commercial 
importance,  inasmuch  as  it  is  used  for  the  preparation  of 
benzaldehyde  (p.  406). 

Benzol  chloride,  C6H5-CHC12,  may  be  obtained  by  treating 
benzaldehyde  with  phosphorus  pentachloride, 

C6H5-CHO  +  PC15  =  C6H5-CHC12  +  POC13, 

but  it  is  prepared  by  chlorinating  toluene  just  as  described 
in  the  case  of  benzyl  chloride,  except  that  the  process  is 
continued  until  twice  as  much  chlorine  has  been  absorbed. 
It  is  a  colourless  liquid,  boiling  at  206°,  and  is  extensively 
used  for  the  preparation  of  benzaldehyde. 

Benzotricliloride,  or  phenylchloroform,  C6H5«CC13,  is  also 
prepared  by  chlorinating  boiling  toluene;  it  boils  at  213°,  and 
when  heated  with  water  it  is  converted  into  benzoic  acid, 

C6H5-CC13  +  2H20  =  C6H5-COOH  +  3HCL 


350  NITRO-COMPOUNDS. 


CHAPTEE    XXII. 

NITRO-COMPOUNDS. 

It  has  already  been  stated  that  one  of  the  most  characteristic 
properties  of  aromatic  compounds  is  the  readiness  with  which 
they  may  be  converted  into  nitro-derivatives  by  the  substitu- 
tion of  nitro-groups  for  hydrogen  of  the  nucleus;  the  com- 
pounds formed  in  this  way  are  of  the  greatest  importance, 
more  especially  because  it  is  from  them  that  the  amido-  and 
diazo-compounds  are  prepared. 

Preparation. — Many  aromatic  compounds  may  be  l  nitrated ' 
— that  is  to  say,  converted  into  their  nitro-derivatives,  by  dis- 
solving them  in  concentrated  nitric  acid  (sp.  gr.  1-3  to  1-5), 
in  the  cold  or  at  ordinary  temperatures,  and  under  such 
conditions  a  mononitro-compound  is  usually  produced ;  ben- 
zene, for  example,  yields  nitrobenzene,  and  toluene,  a  mixture 
of  o-  and  p-nitrotoluenes, 

C6H6  +  HN03  =  C6H5.N02  +  H20 

C6H6.CH8  +  HN03  =  C0H4<^  +  H20. 

Some  aromatic  compounds,  however,  are  insoluble  •  in  nitric 
acid,  and  are  then  only  very  slowly  acted  on ;  in  such  cases, 
a  mixture  of  concentrated  nitric  and  sulphuric  acids  is  used. 
This  mixture  is  also  used  in  many  cases,  even  when  the 
substance  is  soluble  in  nitric  acid,  because  the  sulphuric  acid 
combines  with  the  water  which  is  produced  during  the 
interaction,  and  thus  its  presence  favours  nitration,  just  as 
the  presence  of  dehydrating  agents  favours  the  formation 
of  ethereal  salts  from  a  mixture  of  an  acid  and  an  alcohol. 
When  a  large  excess  of  nitric  and  sulphuric  acids  is  employed, 
and  especially  when  heat  is  applied,  the  aromatic  compound 
is  usually  converted  into  (a  mixture  of  isomeric)  dinitro-  or 
trinitro-derivatives ;  benzene,  for  instance,  yields  a  mixture 


NITROCOMPOUNDS.  351 

of  three  dinitro-benzenes,  the  principal  product,  however, 
being  the  meta-compound, 

C6H6  +  2IDsT03  =  C6H4(N02)2  +  2H20. 

As  soon  as  nitration  is  complete  (portions  of  the  product  may 
be  tested  from  time  to  time),  the  solution  or  mixture,  having 
been  cooled  if  necessary,  is  poured  on  to  ice  or  into  a  large 
volume  of  water,  and  the  product,  which  is  usually  pre- 
cipitated in  crystals,  separated  by  filtration,  or  if  an  oil,  by 
extraction  with  ether,  or  in  some  other  manner. 

Generally  speaking,  the  number  of  hydrogen  atoms  dis- 
placed by  nitro-groups  is  greater  the  higher  the  temperature 
and  the  more  concentrated  the  acid,  or  acid  mixture,  em- 
ployed, but  depends  to  an  even  greater  extent  on  the  nature 
of  the  substance  undergoing  nitration,  because  the  introduc- 
tion of  nitro-groups  is  facilitated  when  other  atoms  or  groups, 
especially  alkyl  radicles,  have  already  been  substituted  for 
hydrogen  of  the  nucleus.  The  nature  of  these  atoms  or  groups 
determines,  moreover,  the  position  taken  up  by  the  entering 
nitro-group;  if  the  former  be  strongly  negative  or  acid  in 
character,  as,  for  example,  -N02,  -COOH,  and  -S03H,  a 
ra-nitro-derivative  is  formed,  whereas,  when  the  atom  or  group 
in  question  is  a  halogen,  an  alkyl,  or  an  amido-  or  hydroxyl- 
group,  a  mixture  of  the  o-  and  ^-nitro-derivatives  is  produced. 

This  directing  influence  of  an  atom  or  group  already  com- 
bined with  the  nucleus,  on  the  position  which  is  taken  up  by 
a  second  atom  or  group,  is  by  no  means  restricted  to  the  case 
of  nitro-compounds,  but  is  observed  in  the  formation  of  all 
benzene  substitution  derivatives,  except,  of  course,  in  that  of 
the  mono-substitution  products ;  so  regularly,  in  fact,  is  this 
influence  exercised,  that  it  is  possible  to  summarise  the  course 
of  those  reactions  which  take  place  in  the  formation  of  the 
best-known  di-derivatives  in  the  following  statements  : 

The  relative  position  taken  up  by  an  atom  or  group,  B, 
depends  on  its  nature,  and  on  that  of  the  atom  or  group,  A, 
already  united  with  the  nucleus. 


352  NITRO-COMPOUNDS. 

When  A  =  Cl,  Br,  I,  NH2,  OH,  CH3, 

and  B  =  Cl,  Br,  N02,  SOSH, 

a  para-compound  is  the  principal  product,  but  it  is  usually 
accompanied  by  smaller  and  varying  quantities  of  the  ortho- 
compound. 

When,  on  the  other  hand, 

A  =  NOjj,  COOH,  S03H,  CHO,  CO.CH3, 
and  B  -  Cl,  Br,  N02,  S03H, 

a  raefa-derivative  is  the  principal  product,  and  only  very 
small  quantities  of  the  ortho-  and  para-compounds  are 
formed. 

These  statements  also  hold  good  when  two  identical  atoms 
or  groups  are  introduced  in  one  operation,  since  the  change 
really  takes  place  in  two  stages ;  when  benzene,  for  example, 
is  treated  with  nitric  acid,  meta-dinitrobenzene  is  the  principal 
product,  whereas  with  bromine  it  yields  para-dibromobenzene. 

Properties. — As  a  rule,  aromatic  nitro-compounds  are 
yellowish,  well-defined  crystalline  substances,  and  are,  there- 
fore, of  great  service  in  identifying  hydrocarbons  and  other 
liquids ;  many  of  them  are  volatile  in  steam,  but,  with  the 
exception  of  certain  mono-nitro-derivatives,  cannot  be  dis- 
tilled under  ordinary  pressure,  as  when  heated  strongly  they 
undergo  decomposition,  sometimes  with  explosive  violence ; 
they  are  generally  insoluble  in  water,  but  soluble  in  benzene, 
ether,  alcohol,  &c.  As  in  the  case  of  the  mtro-paraffin8 
(part  i.  p.  181),  the  nitro-group  is  very  firmly  combined,  and 
is  not,  as  a  rule,  displaced  by  the  hydroxyl-group  on  treat- 
ment with  potash  even  at  high  temperatures. 

The  most  important  reaction  of  the  nitro-compounds — 
namely,  their  behaviour  on  reduction,  is  described  later 
(p.  356). 

Nitrobenzene,  C6H5-N02,  is  usually  prepared  in  the  labora- 
tory by  slowly  adding  to  benzene  (10  parts)  a  mixture  of 
nitric  acid  of  sp.  gr.  1-45  (12  parts),  and  concentrated 
sulphuric  acid  (16  parts),  the  temperature  being  kept  below 
about  40°  by  cooling  in  water,  and  the  mixture  being 


NITRO-COMPOUNDS.  353 

constantly  shaken ;  the  benzene  dissolves  in  the  acids, 
although  it  does  not  appear  to  do  so,  because  it  is  quickly 
converted  into  the  nitro-compoimd,  which  separates  again 
as  a  yellowish-brown  oil.  As  soon  as  all  the  benzene  has 
been  added,  the  mixture  is  heated  at  about  80°  for  half  an 
hour,  then  cooled,  and  poured  into  a  large  volume  of  water ; 
the  nitrobenzene,  which  collects  at  the  bottom  of  the  vessel, 
is  separated  with  the  aid  of  a  funnel,  washed  with  a  little 
water  or  dilute  soda  until  free  from  acid,  dried  with  calcium 
chloride,  and  fractionated,  in  order  to  separate  it  from  un- 
changed benzene  and  from  small  quantities  of  dinitrobenzene 
which  may  have  been  produced ;  this  is  very  easily  accom- 
plished, as  the  boiling-points  of  the  three  compounds  are 
widely  different. 

On  the  large  scale,  nitrobenzene  is  prepared  in  a  similar 
manner,  but  the  operation  is  carried  out  in  iron  vessels  pro- 
vided with  an  arrangement  for  stirring,  and  the  product  is 
distilled  from  iron  retorts,  or,  better,  in  a  current  of  steam. 

Nitrobenzene  is  a  pale-yellow  oil  of  sp.  gr.  1-2  at  0°,  and 
has  a  strong  smell  which  is  very  like  that  of  benzaldehyde 
(p.  406) ;  it  boils  at  205°,  is  volatile  in  steam,  and  is  miscible 
with  organic  liquids,  but  practically  insoluble  in  water;  in 
spite  of  the  fact  that  it  is  poisonous,  it  is  often  employed 
instead  of  oil  of  bitter  almonds  for  flavouring  and  per- 
fuming purposes,  under  the  name  of  'essence  of  mirbane;' 
its  principal  use,  however,  is  for  the  manufacture  of  aniline 
(p.  361). 

Meta-dinitrobenzene,  C6H4(N02)2,  is  obtained,  together 
with  small  quantities  of  the  o-  and  jp-dinitro-compounds,  when 
benzene  is  gradually  added  to  a  mixture  of  nitric  acid  (sp.  gr. 
1-5)  and  concentrated  sulphuric  acid,  and  the  whole  then 
heated  on  a  sand-bath,  until  a  portion  of  the  oil,  which  floats 
on  the  surface,  solidifies  completely  when  dropped  into  water; 
after  cooling,  the  mixture  is  poured  into  a  large  volume  of 
water,  the  solid  product  separated  by  filtration,  washed  with 
water,  and  recrystallised  from  hot  alcohol  until  its  melting- 

W 


354  NITRO-COMPOUNDS. 

point  is  constant ;  the  o-  and  ^-compounds,  formed  only  in 
very  small  quantities,  remain  dissolved  in  the  mother-liquors. 

Meta-dinitrobenzene  crystallises  in  pale-yellow  needles,  melts 
at  90°,  and  is  volatile  in  steam ;  it  is  only  sparingly  soluble 
in  boiling  water,  but  dissolves  freely  in  most  organic  liquids. 
On  reduction  with  alcoholic  ammonium  sulphide  (p.  357)  it 
is  first  converted  into  m-nitraniline  (p.  363),  and  then  into 
m-phenylenediamine  or  meta-diarnidobenzene,  C6H4(NH2)2  (p. 
364). 

o-Dinitrolenzene  and  p-dinitrobenzene  are  colourless,  crystal- 
line compounds,  melting  at  118°  and  173°  respectively;  they 
resemble  the  corresponding  ?7z-compound  in  their  behaviour 
on  reduction,  and  in  most  other  respects.  o-Dinitrobenzene, 
however,  differs  notably  from  the  other  two  isomerides,  inas- 
much as  it  interacts  with  boiling  soda,  yielding  o-nitrophenol 
(p.  393),  and,  with  alcoholic  ammonia  at  moderately  high 
temperatures,  giving  o-nitraniline  (p.  363).  A  similar  be- 
haviour is  observed  in  the  case  of  other  o-dinitro-compounds, 
the  presence  of  the  one  nitro-group  rendering  the  other  more 
easily  displaceable. 

Symmetrical  trinitrobenzene,  C6H3(N02)3,  is  formed  when 
the  ra-dinitro-compound  is  heated  with  a  mixture  of  nitric  and 
anhydrosulphuric  acids ;  it  crystallises  in  colourless  plates  and 
needles,  melting  at  121-122°. 

The  halogen  derivatives  of  benzene  are  readily  nitrated, 
yielding,  however,  the  o-  and  jo-mononitro-derivatives  only, 
according  to  the  general  rule ;  the  w-nitro-halogen  compounds 
are  therefore  prepared  by  chlorinating  or  brominating  nitro- 
benzene. All  these  nitro-halogen  derivatives  are  crystalline, 
and,  as  will  be  seen  from  the  following  table,  their  melting- 
points  exhibit  the  regularity  mentioned  above  (p.  332),  except 
in  the  case  of  ?7i-iodonitrobenzene  : 

Ortho.         Meta.          Para. 

Chloronitrobenzene,     C6H4C1-N02,      32-5°      44-4°        83° 
Bromonitrobenzene,      C6H4Br.N02,      41-5       56  126 

lodonitrobenzene,        CgH-NO          49  33  171 


NITRO-COM  POUNDS.  355 

They  are,  on  the  whole,  very  similar  in  chemical  properties, 
except  that,  as  already  pointed  out  (p.  346),  the  o-  and  p- 
compounds  differ  from  the  w-compounds  in  their  behaviour 
with  alcoholic  potash  and  ammonia,  a  difference  which  recalls 
that  shown  by  the  three  dinitrobenzenes. 

The  nitrotoluenes,  C6H4(CH3)-X02,  are  important,  because 
they  serve  for  the  preparation  of  the  toluidines  (p.  364).  The 
o-  and  ^-compounds  are  prepared  by  nitrating  toluene,  and  may 
be  partially  separated  by  fractional  distillation ;  o-nitrotoluene 
is  liquid  at  ordinary  temperatures,  and  boils  at  223°,  whereas 
p-nitrotolmne  is  crystalline,  and  boils  at  237°,  its  melting-point 
being  54°.  m-Nitrotoluene  is  not  easily  prepared ;  it  melts 
at  16°,  and  boils  at  230°. 

Many  other  nitro-compounds  are  mentioned  later. 


CHAPTER    XXIII. 

AMIDOCOMPOUNDS    AND    AMINES. 

The  hydrogen  atoms  in  ammonia  may  be  displaced  by 
aromatic  radicles,  bases,  such  as  aniline,  C6H5-NH2,  benz- 
ylamine,  C6H3-CH2-NH2,  and  diamidobenzene,  C6H4(NH2)2, 
which  are  analogous  to,  and  have  many  properties  in  common 
with  the  fatty  amines,  being  produced;  as,  however,  those 
compounds  which  contain  the  amido-group  directly  united  with 
carbon  of  the  nucleus  differ  in  many  important  respects  from 
those  in  which  this  group  is  present  in  the  side-chain,  the 
former  are  usually  called  amido-compounds,  whereas  the  latter 
are  classed  as  aromatic  amines,  because  they  are  the  true 
analogues  of  the  fatty  amines. 

Amido-compounds. 

The  amido-compounds  may,  therefore,  be  regarded  as 
derived  from  benzene  and  its  homologues  by  the  substitution 
of  one  or  more  amido-groups  for  hydrogen  atoms  of  -the 


3t>6  AMIDO-COMPOUNDS    AND    AMINES. 

nucleus  ;  they  may  be  classed  as  mono-,  di-,  tri-,  &c.,  amido- 
coinpounds,  according  to  the  number  of  such  groups  which 
they  contain. 


C6H6.NH2  CeH4(NH2)2  C6H3(NH2)3. 

Amidobenzene  (Aniline).         Diamidobenzene.  Triamidobenzene. 

With  the  exception  of  aniline,  all  amido-compounds  exist  in 
three  or  more  isomeric  modifications;  there  are,  for  example, 
three  isomeric  (o.m.p.)  diamidobenzenes,  and  three  isomeric 
(o.m.p.)  amidotoluenes,  or  toluidines,  C6H4(CH3)-NH2,  a 
fourth  isomeride  of  the  toluidines  —  namely,  benzylamine, 
C6H5.CH2.NH2  (p.  368),  being  also  known. 

Preparation.  —  The    amido-compounds   are    almost    always 
prepared  by  the  reduction  of  the  nitro-com  pounds  ;    various 
reducing  agents,  such  as  tin,  zinc,  or  iron,  and  hydrochloric  or 
acetic  acid,  are  employed,  but  perhaps  the  most  common  one 
is  a  solution  of  stannous  chloride  in  hydrochloric  acid, 
C6H5-N02  +  6H  -  C6H5-NH2  +  2H2O 
C6H4<         +  6H  =  C6H4<          +  2H20 


C6H5.N02  +  3SnCl2  +  6HC1  =  C6H5-NH2  +  3SnCl4  +  2H20. 

Reduction  is  usually  effected  by  simply  treating  the  nitre-com- 
pound with  the  reducing  mixture  without  a  special  solvent,  when  a 
vigorous  reaction  often  ensues,  heating  being  seldom  necessary 
except  towards  the  end  of  the  operation.  The  solution  contains  the 
amido-compound,  combined  as  a  salt  with  the  acid  which  has  been 
employed  ;  when,  however,  tin  or  stannous  chloride  and  hydro- 
chloric acid  have  been  used,  a  double  salt  of  the  hydrochloric!  e  of 
the  base  and  stannic  chloride  is  produced  ;  in  the  reduction  of  nitro- 
benzene, for  example,  the  double  salt,  aniline  stannichloride,  has 
the  composition 

(C6H5.NH2,  HC1)2,  SnCl4. 

In  any  case,  the  salt  is  decomposed  by  the  addition  of  excess  of 
caustic  soda  or  lime,  and  the  liberated  base  either  distilled  with 
steam  or  extracted  with  ether,  or  isolated  in  some  other  manner 
suitable  to  the  special  case.  Recent  researches  show  that  the  re- 
duction of  nitro-compounds  may  take  place  in  two  stages:  in  the 
first  place,  a  derivative  of  hydroxylamine  is  produced, 
C6H5-N02  +  4H  =  C6H5.NH-OH  +  H20, 

Plienylhydroxylainine, 


AMIDO-COMPOUNDS   AND   AMINES.  357 

and  this,  by  the  further  action  of  the  reducing  agent,  is  converted 
into  the  aniido-compound. 

Nitro-compounds  may  also  be  reduced  to  amido-compounds 
by  employing  sulphuretted  hydrogen  in  alkaline  solution,  or, 
more  conveniently,  an  alcoholic  solution  of  ammonium 
sulphide, 

C6H6-N02  +  3SH2  =  C6H5.NH2  +  2H20  +  3S. 

The  nitro-compound  is  dissolved  in  alcohol,  concentrated 
ammonia  added,  and  a  stream  of  sulphuretted  hydrogen  passed 
into  the  solution,  until  reduction  is  complete,  heat  being  applied  if 
necessary.  The  solution  is  then  filtered  from  precipitated  sulphur, 
the  alcohol  distilled  off,  and  the  residue  acidified  with  hydro- 
chloric acid ;  the  filtered  solution  of  the  hydrochloride  of  the  base 
is  now  evaporated  to  a  small  bulk  and  treated  with  soda,  when  the 
base  separates  as  an  oil  or  solid,  and  may  then  be  purified  by  dis- 
tillation, recrystallisation,  &c. 

When  there  are  two  or  more  nitro-groups  in  a  compound, 
partial  reduction  may  be  accomplished  either  by  treating  its 
alcoholic  solution  with  the  calculated  quantity  of  stannous 
chloride  and  hydrochloric  acid,  or  by  adding  strong  ammonia 
and  passing  sulphuretted  hydrogen ;  in  the  latter,  as  in  the 
former  case,  one  nitro-group  is  reduced  before  a  second  is 
attacked,  so  that  by  stopping  the  current  of  gas  at  the  right 
time  (usually  ascertained  by  weighing  the-sulphuretted  hydrogen 
absorbed),  only  partial  reduction  takes  place.  Dinitrobenzene, 
for  example,  can  be  converted  into  nitraniline  by  either  of 
these  methods,  the  latter  being  the  mdre  convenient, 

\S          CH  +  6H  =  CH°»  +  2H0. 


The  amido-derivatives  of  toluene,  xylene,  &c.,  are  com- 
mercially prepared  by  heating  the  hydrochlorides  of  the 
isomeric  alkylanilines,  such  as  methylaniline  and  dimethyl- 
aniline,  at  280-300°,  when  the  alkyl-group  leaves  the  nitrogen 
atom  and  enters  the  nucleus  (compare  p.  365), 


C6H5-NH-CH3,  HC1  =  C6H4< 

Methylaniline  Hydrochloride.        p-Toluidine  Hydrochloride. 


358  AMIDO-COMPOUNDS   AND   AMINES. 

In  the  case  of  dimethylaniline  tins  change  takes  place  in 
two  stages, 

C6H6-N(CH3)2,  HC1     =     ^H^NI^QII  ,  HC1 

Dimethylaniline  Hydrochloride.  Methyl-p-toluidine  Hydrochloride. 

PH"  /         3 

CaM/C  Tkr-rr  r^TT    Tjr<i  ==  CflHqv—  C 

'HH 


qv—      » 
\NH2,HC1. 

Methyl-p-toluidine  Hydrochloride.  Xylidine  Hydrochloride 

[CH3:CH3:NH2  =  1:3:4]. 

In  this  remarkable  reaction  the  alkyl-group  displaces  hydrogen 
from  the  ortho-,  and  from  the  para-position  to  the  amido-group,  but 
principally  the  latter  ;  meta-derivatives  cannot  be  prepared  in  this 
way. 

This  method  is  used,  on  the  large  scale,  for  preparing  toluidine, 
xylidine,  &c.  ;  aniline  is  heated  with  methyl  alcohol  and  hydro- 
chloric acid  at  a  high  temperature,  when  the  methyl-  and  dimethyl- 
anilines  first  produced  (p.  365)  undergo  intramolecular  change  as 
explained  above. 

The  diamido-compounds,  such  as  the  o-,  ra-,  and  ^9-diamido- 
benzenes  or  phenylenediamines,  C6H4(NH2)2,  are  prepared 
by  reducing  either  the  corresponding  dinitrobenzenes, 
C6H4(N02)2,  or  the  nitranilines,  C6H4(N02)-NH2,  generally 
with  tin  and  hydrochloric  acid. 

Properties.  —  The  monamido-compoimds  are  mostly  colour- 
less liquids,  which  distil  without  decomposition,  and  are 
specifically  heavier  than  water  ;  they  have  a  faint  but  charac- 
teristic odour,  and  dissolve  freely  in  alcohol,  ether,  and  other 
organic  solvents,  but  they  are  only  sparingly  soluble  in  water  ; 
on  exposure  to  light  they  rapidly  darken,  and  ultimately 
become  brown  or  black. 

They  are  comparatively  weak  bases,  which  are  neutral 
to  litmus,  and  although  they  combine  with  acids  to  form 
salts  such  as  aniline  hydrochloride,  C6H5-NH2,  HC1,  these 
salts  are  readily  decomposed  by  weak  alkalies  or  alkali 
carbonates,  with  liberation  of  the  bases  ;  in  these  respects, 
then,  the  amido-com  pounds  differ  in  a  marked  manner  from 
the  strongly  basic  fatty  amines  and  from  the  true  aromatic 
amines,  such  as  benzylamine  (p.  368). 


AMIDO -COMPOUNDS    AND    AMINES.  359 

The  feebly  basic  character  of  the  amido-compounds  is  due 
to  the  fact  that  the  phenyl  radicle,  C6H5-,  has  a  marked 
negative  or  acid  character,  and  its  substitution  for  one  of  the 
hydrogen  atoms  in  ammonia  has  the  effect  of  diminishing  or 
neutralising  the  basic  character  of  the  latter,  a  result  which 
is  directly  the  opposite  of  that  arrived  at  by  displacing 
the  hydrogen  atoms  of  ammonia  by  an  alkyl  (or  positive) 
group,  since  the  amines  are  stronger  bases  than  ammonia. 

When  two  hydrogen  atoms  in  ammonia  are  displaced  by  phenyl- 
groups,  as  in  diphenylamine,  (C6H5)2NH  (p.  367),  a  still  feebler 
base  is  produced,  the  salts  of  which  are  decomposed  by  water. 
Triphenylamine,  (C6H5)3N  (p.  368),  moreover,  does  not  form  salts  at 
all. 

For  the  same  reason  the  hydroxy-,  nitro-,  and  halogen-derivatives 
of  the  amido-compoimds,  such  as  amido-phenol,  C6H4(OH)-NH2, 
nitraniline,  C6H4(N02)-NH2,  chloraniline,  C6H4C1'NH2,  &c.,  are  even 
weaker  bases  than  the  amido-compounds  themselves,  because  the 
presence  of  the  negative  groups  or  atoms,  HO-,  N02-,  C1-,  &c., 
enhances  the  acid  character  of  the  phenyl  radicle. 

The  amido-compounds  also  differ  from  the  fatty  primary 
amines  and  from  the  true  aromatic  primary  amines  in  their 
behaviour  with  nitrous  acid.  Although  when  warmed  with 
nitrous  acid  in  aqueous  solution  they  yield  phenols  by  the 
substitution  of  hydroxy  1  for  the  amido-group,  just  as  the 
fatty  amines  under  similar  treatment  are  converted  into 
alcohols  (part  i.  p.  202), 

C6H5.NH2  +  N02H  =  C6H5.OH  +  N0  +  H20 
C2H5-NH,  +  N02H  =  C2H5-OH  +  N2  +  H20, 

yet  when  treated  with  nitrous  acid  in  cold  aqueous  solution, 
they  are  converted  into  diazo-com  pounds  (p.  370),  substances 
which  cannot  be  produced  from  the  primary  amines. 

It  will  be  evident  from  the  above  statements  that  there  are 
several  important  differences  between  the  amido-compounds 
and  the  true  primary  amines,  the  character  of  an  amido- 
group  in  the  nucleus  being  influenced  by  its  state  of  combina- 
tion; nevertheless,  except  as  regards  those  points  already 


360  AMIDO-COMPOUNDS    AND    AMINES. 

mentioned,  amido-compounds  have,  on  the  whole,  properties 
very  similar  to  those  of  the  true  primary  amines. 

The  amido-compounds,  like  the  primary  amines,  interact 
readily  with  alkyl  halogen  compounds,  yielding  alkyl-deriva- 
tives,  such  as  methylaniline,  C6H5-NH-CH3,  dimethylaniline, 
C6H5'N(CH3)2,  &c.,  and  also  compounds  such  as  phenyl- 
trimethylammonium  iodide,  C6H5-N(CH3)3I,  which  corre- 
spond with  the  quaternary  ammonium  salts  (part  i.  p.  205). 

They  are  also  readily  acted  on  by  anhydrides  and  acid 
chlorides,  and  even  by  acids  on  prolonged  heating,  yielding 
substances  such  as  acetanilide  and  acetotoluidide,  which  are 
closely  allied  to  the  fatty  amides  (part  i.  p.  161),  and  from 
which  they  may  be  regarded  as  derived, 

C6H5.N£2  +  CH3.COOH  =  C6H5.NH-CO.CH3  +  H20 
C6H4(CH8).NH2  +  (CH3.CO)20  = 

p-Toluidine. 

C6H4(CH3)-ira.CO.CH3  +  CH3.COOH  ; 

Aceto-p-toluidide. 

these  compounds,  like  the  amides,  are  readily  resolved  into 
their  constituents  on  boiling  with  acids  or  alkalies, 

CsH<<NH3.CO.CH3  +  H*°  =  CA<£T%  +  CH3.COOH. 

The  amido-compounds,  like  the  fatty  primary  amines,  give 
the  carbylamine  reaction  ;  when  a  trace  of  aniline,  for  example, 
is  heated  with  alcoholic  potash  and  chloroform,  an  intensely 
nauseous  smell  is  observed,  due  to  the  formation  of  phenyl- 
carbylamine  (part  i.  pp.  173,  202), 

CHC1  +  3KOH  =  CH-N:  C  +  3KC1  +  3H0. 


Aqueous  solutions  of  amido-compounds  are  coloured  in- 
tensely violet  on  the  addition  of  a  solution  of  bleaching-powder 
or  sodium  hypochlorite,  a  behaviour  which,  as  well  as  the 
carbylamine  reaction,  is  made  use  of  in  their  detection. 

Diamido-  and  triamido-compounfa,  such  as  the  three  (o.m.  p.) 
phenylenediamines  or  diamidobenzenes,  C6H4(NH2)2,  and 
the  triamidobenzenes,  C6H3(NH9)3,  are  very  similar  to  the 


AMIDO-COMPOUNDS    AND    AMINES.  361 

monamido-compounds  in  chemical  properties,  but  differ  from 
them  usually  in  being  solid,  more  readily  soluble  in  water,  and 
less  volatile;  since,  moreover,  they  contain  two  and  three 
amido-groups  respectively,  they  neutralise  two  or  three  equi- 
valents of  an  acid,  yielding  salts  such  as  C6H4(NH9)2,  2HC1 
and  C6H3(NH2)3,  3HC1. 

Aniline  and  its  Derivatives. 

Aniline,  amidobenzene,  or  phenylamine,  C6H5-NH2,  was 
first  prepared  by  Unverdorben  in  1826  by  distilling  indigo,  the 
name  aniline  being  derived  from  'anil,'  the  Spanish  for 
indigo.  Runge  in  1834  showed  that  aniline  is  contained  in 
small  quantities  in  coal-tar,  but  its  preparation  from  nitro- 
benzene was  first  accomplished  by  Zinin  in  1841. 

Aniline  is  manufactured  on  a  very  large  scale  by  the  reduc- 
tion of  nitrobenzene  with  scrap  iron  and  crude  hydrochloric 
acid  ;  but  in  preparing  small  quantities  in  the  laboratory,  the 
most  convenient  reducing  agent  is  tin  and  hydrochloric  acid, 

C6H5.N09  +  6H  -  C6H5.NH2  +  2H00, 
2C6H5-N02  +  3Sn  + 1 2HC1  =  2C6H5-NH2  +  3SnCl4  +  4H2O. 

Nitrobenzene  (50  grams)  and  granulated  tin  (80  grams)  are  placed 
in  a  flask,  and  concentrated  hydrochloric  acid  (290  grams)  added 
in  small  quantities  at  a  time  ;  at  first  the  mixture  must  be  cooled  if 
the  reaction  be  too  violent,  but  when  all  the  acid  has  been  added, 
the  product  is  carefully  heated  on  a  water-bath  for  about  half  an 
hour.  The  solution  of  aniline  stannicliloride  is  now  treated  with 
soda  until  strongly  alkaline,  the  liberated  aniline  distilled  in  steam, 
and  the  distillate  extracted  with  ether.  The  ethereal  extract  is 
then  dried  over  solid  potash,  the  ether  distilled  off,  and  the  aniline 
purified  by  distillation. 

Aniline  is  a  colourless  oil,  boiling  at  183° ;  it  has  a  faint, 
characteristic  odour,  and  is  sparingly  soluble  in  water,  but 
readily  in  alcohol  and  ether ;  it  gradually  turns  yellow  when 
exposed  to  light  and  air,  becoming  ultimately  almost  black. 
Although  neutral  to  litmus,  aniline  has  very  decided  basic 
properties,  and  neutralises  acids,  forming  soluble  salts,  such  as 


362  AMIDO-COMPOUNDS   AND   AMINES. 

aniline  hydrochloride,  C6H5-NH2,  HC1,  and  the  rather  spar- 
ingly soluble  sulphate,  (C6H5-NH2)2,  H2S04.  The  former,  like 
the  hydrochlorides  of  ethylamine,  &c.,  forms  double  salts  with 
platinum  chloride  and  gold  chloride  ;  on  treating  a  moderately 
concentrated  solution  of  the  hydrochloride  with  platinum 
chloride,  for  example,  the  platinochloride, 

(C6H5.NH2)2,  H2PtCl6, 

is  precipitated  in  yellow  plates,  which  are  moderately  soluble 
in  water. 

When  aniline  is  heated  with  chloroform  and  alcoholic 
potash,  it  yields  phenylcarbylamine,  C6H5-N:C,  a  substance 
readily  recognised  by  its  penetrating  and  very  disagreeable 
odour ;  the  presence  of  aniline  may  also  be  detected  by  treat- 
ing its  aqueous  solution  with  bleaching-powder  solution  or 
sodium  hypochlorite,  when  an  intense  purple  colouration  is 
produced. 

When  solutions  of  the  salts  of  aniline  are  treated  with 
nitrous  acid,  at  ordinary  temperatures,  salts  of  diazo-com- 
pounds  (p.  370)  are  formed,  but  on  warming,  the  latter  are 
decomposed  with  formation  of  phenol  (p.  391). 

Aniline  is  very  largely  employed  in  the  manufacture  of 
dyes,  and  in  the  preparation  of  a  great  number  of  benzene 
derivatives. 

Acetanilide,  C6H5.NH.CO.CH3,  is  readily  prepared  by 
boiling  aniline  with  excess  of  glacial  acetic  acid  on  a  reflux 
apparatus  for  several  hours,  when  the  aniline  acetate  first 
formed  is  slowly  converted  into  acetanilide,  with  elimination 
of  water.  The  product  is  purified  by  fractionation  or  simply 
by  recrystallisation  from  boiling  water, 

C6H5.NH2,  CH3-COOH  =  C6H5-NH-CO-CH3  +  H20. 

It  crystallises  in  glistening  plates,  melts  at  115°,  and  is 
sparingly  soluble  in  cold  water,  but  readily  in  alcohol ;  when 
treated  with  acids  or  alkalies,  it  is  rapidly  hydrolysed  into 
aniline  and  acetic  acid.  It  is  used  in  medicine  as  a  febrifuge, 
under  the  name  of  antifebrin. 


AMIDO-COMPOUNDS   AND   AMINES.  363 

Formanilide,  C6H5-KH.CHO,  the  anilide  of  formic  acid, 
and  oxanilide,  C6H5.NH.CO-CO-NH-C6H5,  the  anilide  of 
oxalic  acid,  may  be  similarly  prepared. 

Substitution  Products  of  Aniline. — Aniline  and,  in  fact,  all  amido- 
coinpounds  are  much  more  readily  attacked  by  halogens  than  the 
hydrocarbons  :  when  aniline,  for  example,  is  treated  with  chlorine 
or  bromine  in  aqueous  solution,  it  is  at  once  converted  into  trichlor- 
aniline,  C6H2C13-NH2,  and  tribromaniline,  C6H2BiyNH2,  respect- 
ively, so  that  in  order  to  obtain  mono-  and  di-substitution  products, 
indirect  methods  must  be  employed. 

The  o-  and  p-chlor -anilines,  C6H4C1-NH2,  may  be  prepared  by 
passing  chlorine  into  acetanilide,  the  ^-derivative  being  obtained 
in  the  larger  quantity.  The  two  isomerides  are  first  separated  by 
crystallisation,  and  then  decomposed  by  boiling  with  an  alkali  or 
acid, 

C6H4C1.NH.CO.CH3  +  KOH  =  C6H4C1-NH2  +  CH3-COOK. 

Chloracetanilide.  Chloraniline. 

The  effect  of  introducing  an  acetyl-group  into  the  amido-group  is 
therefore  to  render  aniline  less  readily  attacked  ;  acetanilide,  in  fact, 
behaves  towards  chlorine  and  bromine  more  like  benzene  than 
aniline.  w-Chloraniline  is  most  conveniently  prepared  by  the 
reduction  of  wi-chloronitrobenzene,  C6H4C1-NO2  (a  substance  formed 
by  chlorinating  nitrobenzene  in  the  presence  of  antimony  chloride). 
o-Chloraniline  and  m-chloraniline  are  oils  boiling  at  207°  and  230° 
respectively,  but  p-chloraniline  is  a  solid,  which  melts  at  69°,  and 
boils  at  231°. 

Nitranilines,  C6H4(N02)-NH2,  cannot  be  obtained  by  nitrat- 
ing aniline,  as  the  nitrous  acid,  produced  by  the  reduction 
of  the  nitric  acid,  converts  the  amido-  into  the  hydroxyl-group, 
and  nitre-derivatives  of  phenol  are  formed. 

The  o-  and  ^9-compounds  are  prepared  by  nitrating  acet- 
anilide, the  o-  and  ^5-nitracetanilides  thus  obtained  being  separ- 
ated by  fractional  crystallisation,  and  then  converted  into  the 
corresponding  nitranilines  by  heating  with  alkalies.  m-Mtran- 
iline  is  very  readily  prepared  by  the  partial  reduction  of 
?«-dinitrobenzene,  CgH^NO^,  with  ammonium  sulphide 
(p.  357). 

o-Xitraniline  melts  at  71°,  m-  at  114°,  and  p-  at  147°;  they 
are  all  sparingly  soluble  in  water,  readily  in  alcohol,  and  on 


364  AMIDO  -COMPOUNDS    AND    AMINES. 

reduction  they  yield  the  corresponding  o-,  ra-,  and  j;-phenyl- 
enediamines, 


6H  =  C  +  2H20. 


Homologues  of  Aniline.  —  The  toluidines,  or  amido-toluenes, 
C6H4(CH3)-NH2,  are  prepared  by  reducing  the  corresponding 
o-t  m-,  and  ^-nitrotoluenes  (p.  355),  by  means  of  tin  and 
hydrochloric  acid,  the  details  of  the  process  being  exactly 
similar  to  those  already  given  in  the  case  of  the  preparation 
of  aniline  from  nitrobenzene, 

C«H«<N§  +  6H  =  C^<NH32  +  2H*°' 

the  o-  and  jp-compounds  may  also  be  prepared  from  methyl- 
aniline  (p.  357).  Both  o-  and  ?^-toluidine  are  oils  boiling 
at  197°,  but  ^-toluidine  is  crystalline,  and  melts  at  45°,  boil- 
ing at  198°.  When  treated  with  nitrous  acid,  the  tolui- 
dines yield  diazo-salts,  from  which  the  corresponding  cresols, 
C6H4(CH3)-OH,  are  obtained,  and  in  all  other  reactions  they 
show  the  greatest  similarity  to  aniline  ;  o-  and  ^-toluidine  are 
largely  employed  in  the  manufacture  of  dyes. 

Diamidobenzenes.  —  The  phenylenediamines,  C6H4(NH2)2, 
are  obtained  by  the  reduction  of  the  corresponding  dinitro- 
benzenes,  or  the  nitranilines,  and  a  general  description  of 
their  properties  has  already  been  given  (p.  361);  o-phenylene- 
diamine  melts  at  103°,  the  m-  and  ^-compounds  at  63°  and 
140°  respectively.  m-Phenylenediamine  gives  an  intense 
yellow  colouration  with  a  trace  of  nitrous  acid,  and  is 
employed  in  water-analysis  for  the  estimation  of  nitrites  ; 
both  the  m-  and  ^-compounds  are  largely  employed  in  the 
manufacture  of  dyes. 

Alkylanilines. 

Those  derivatives  of  the  amido-compounds,  obtained  by 
displacing  one  or  both  of  the  hydrogen  atoms  of  the  amido- 
group  by  alkyl  radicles,  are  substances  of  considerable  import- 
ance, and  are  usually  known  as  alkylanilines.  They  are 


AMIDO-COMPOUNDS    AND    AMINES.  365 

prepared  by  heating  the  amido-compounds,  for  some  hours, 
with  the  alkyl  halogen  compounds,  the  reaction  being  analo- 
gous to  that  which  occurs  in  the  formation  of  secondary  and 
tertiary  from  primary  amines  (p.  369), 

C6H5-NH2  +  RC1  =  C6H5-NHE,  HC1 
C6H5-]STH2  +  2RC1  =  C6H5.NR2,  HC1  +  HC1. 
Instead  of  employing  the  alkyl  halogen  compounds,  a  mixture 
of  the  corresponding  alcohol  and  halogen  acid  may  be  used ; 
methyl-  and  dimethyl-aniline,  for  example,  are  prepared,  on 
the  large  scale,  by  heating  aniline  with  methyl  alcohol  and 
hydrochloric  acid  at  200-250°, 

C6H5-NH2,  HC1  +  CH3.OH  =  C6H5.NH(CH3),  HC1  +  H20 
C6H5.NH2,HC1  +  2CH3-OH  =  C6H5-N(CH3)2,  HC1  +  2H20. 

In  either  case  the  product  consists  of  the  salts  of  the  mono-  and 
dialkyl-derivatives,  mixed  with  certain  quantities  of  unchanged 
substances,  hut  the  mono-alkyl  derivative  is  usually  present  in 
small  quantity  only  (about  5  per  cent.).  The  three  bases  are 
separated  as  follows :  The  product  is  treated  with  potash,  and  the 
free  bases  (aniline,  methylaniline,  and  dimethylaniline),  which 
separate  as  an  oily  layer,  are  extracted  with  ether.  After  distilling 
off  the  ether,  the  mixture  is  digested  for  a  short  time  with  acetic 
anhydride,  by  which  treatment  the  aniline  and  methylaniline  are  con- 
verted into  acetanilide,  C6Hj-NH-CO-CH3,  and  methylacetanilide, 
C6H5-X(CH3)-CO-CH3,  respectively,  whereas  the  dimethylaniline  is 
not  acted  on;  the  whole  is  then  distilled,  the  portion  boiling  below 
175°,  which  consists  of  acetic  anhydride,  being  collected  separately. 
The  crystals,  which  are  deposited  on  standing  from  the  portions 
passing  over  above  175°,  are  separated  by  nitration  and  pressure 
from  the  oily  dimethylaniline,  which  is  then  purified  by  frac- 
tionation. 

After  washing  the  crystalline  anilides  with  very  dilute  acetic 
acid,  the  mixture  is  hydrolysed  with  hydrochloric  acid,  the  liquid 
diluted  considerably  with  water,  cooled,  and  an  excess  of  sodium 
nitrite  added ;  the  aniline  is  thus  converted  into  diazobenzene 
chloride  (p.  370),  and  the  methylaniline  into  nitrosomethylaniline, 
C6H5.N(CH3).NO.  The  latter  is  extracted  with  ether,  reduced 
with  tin  and  hydrochloric  acid  (p.  366),  and  the  regenerated  methyl- 
aniline  purified  by  distillation  in  steam  and  fractionation.  Ethyl- 
and  dicthyl -aniline  may  be  prepared  and  isolated  in  a  similar 
manner. 


366  AMIDO-COMPOUNDS    AND    AMINES. 

These  mono-  and  di-alkyl  derivatives  are  stronger  bases 
than  the  amido-eompounds  from  which  they  are  derived,  the 
presence  of  the  positive  alkyl-group  counteracting  to  some 
extent  the  action  of  the  negative  phenyl-group  (compare  p. 
359)  ;  they  are,  in  fact,  very  similar  in  properties  to  the 
secondary  and  tertiary  amines  respectively,  and  may  be 
regarded  as  derived  from  the  fatty  'amines  by  the  substitution 
of  a  phenyl-group  for  a  hydrogen  atom,  just  as  the  secondary 
and  tertiary  amines  are  obtained  by  displacing  hydrogen 
atoms  by  alkyl-groups.  Methylaniline,  for  example,  is  also 
phenylmethylamine,  and  its  properties  are  those  of  a  sub- 
stitution product  of  methylamine. 

The  mono-alkylanilines,  like  the  secondary  amines,  are 
converted  into  yellowish  nitroso-compounds  on  treatment 
with  nitrous  acid, 

C6H5.NH-CH3  +  HO-NO  =  C6H6.N(NO).CH8  +  H20. 

Methylaniline.  Nitrosomethylaniline. 


(CH3)2NH  +  HO-NO  =  (CH3)2.N.NO  +  H20. 

Dimethylamine.  Nitrosodimethylamine. 

0 

These  nitroso-compounds  give  Liebermann's  nitroso-reaction 
(part  i.  p.  204),  and  on  reduction  they  yield  ammonia  and 
the  original  alkylaniline, 

C6H5.N(NO)-CH3  +  6H  =  C6H5.NH-CH3  +  NH3  +  H2O. 

Methylaniline,  C6H5-NH-CH3,  prepared  as  just  described, 
is  a  colourless  liquid  which  boils  at  191°,  and,  compared  with 
aniline,  has  strongly  basic  properties.  On  adding  sodium 
nitrite  to  its  solution  in  hydrochloric  acid,  nitrosomethyl- 
aniline,  C6H5-N(NO)-CH3,  is  precipitated  as  a  light-yellow 
oil. 

Dimethylaniline,  C6H5-N(CH3)2,  the  preparation  of  which 
has  just  been  given,  is  a  colourless,  strongly  basic  oil,  which 
boils  at  192°;  it  is  largely  used  in  the  manufacture  of  dyes. 

The  di-alkylanilines,  such  as  dimethylaniline,  C6H5-N(CH3)2,  also 
interact  readily  with  nitrous  acid  (a  behaviour  which  is  not  shown 
by  tertiary  fatty  amines),  intensely  green  (iso)nitroso-compounds 


AMI  DO-COMPOUNDS    AND    AMINES.  367 

being  formed,  the  NO-  group  displacing  hydrogen  of  the  nucleus 
from  the  p- position  to  the  nitrogen  atom, 

C6H5.N(CH3)2  +  HO-NO  =  C6H4<g°  g^  +  H20. 

Nitrosodimethylaniline. 

These  substances  do  not  give  Liebermann's  nitroso-reaction,  and 
when  reduced  they  yield  derivatives  of  ^-phenylenediamine, 

+  4H  =  C6H4<N(C2H3)2  +H2O. 

Dimethyl-p-phenylenediamine. 

p  -  Nitrosodimethylaniline,  C6H4<C\^//-iTT  \  ,  is  prepared  by  dis- 
solving dimethylaniline  (1  part)  in  water  (5  parts),  and  concen- 
trated hydrochloric  acid  (2-5  parts),  and  gradually  adding  to  the 
well-cooled  solution  the  calculated  quantity  of  sodium  nitrite 
dissolved  in  a  little  water.  The  yellow  crystalline  precipitate  of 
nitrosodimethylaniline  hydrochloride  is  separated  by  filtration, 
dissolved  in  water,  decomposed  by  potassium  carbonate,  and  the 
free  base  extracted  with  ether.  Nitrosodimethylaniline  crystal- 
lises from  ether  in  dark-green  plates,  and  melts  at  85° ;  it  is  not  a 
nitrosamine,  and  does  not  give  Liebermann's  nitroso-reaction. 
When  reduced  with  zinc  and  hydrochloric  acid,  it  is  converted 
into,  dimethyl -p-pheny  I  enediainine  (see  above),  and  when  boiled 
with  dilute  soda,  it  is  decomposed  into  quinone  monoxime 
(p-nitrosophenol)  and  dimethylamine, 

2O  =  C6H4<^  +  NH(CH3)2. 


DipJienylamine  and  Triphenylamine. 

The  hydrogen  atoms  of  the  amido-group  in  aniline  may 
also  be  displaced  by  phenyl  radicles,  the  compounds  diphenyl- 
amine,  (C6H5)2NH,  and  triphenylamine,  (C6H5)3N,  being  pro- 
duced. These  substances,  however,  cannot  be  obtained  by 
treating  aniline  with  chlorobenzene,  C6H5C1,  a  method  which 
would  be  analogous  to  that  which  is  employed  in  the  prepara- 
tion of  diethylamine  and  triethylamine,  because  the  halogen 
is  so  firmly  bound  to  the  nucleus,  that  no  action  takes  place 
even  when  the  substances  are  heated  together. 

Diplienylamine  is  most  conveniently  prepared  by  heating 


368  AMIDO-COMPOUNDS    AND   AMINES. 

aniline  hydrochloride  with  aniline  at  about  240°  in  closed 
vessels, 


C6H5.NH2,  HC1  +  C6H5-NH2  =  (C6H5)2-NH  +  NH4C1. 

It  is  a  colourless,  crystalline  substance,  which  melts  at  54°, 
boils  at  310°,  and  is  insoluble  in  water,  but  readily  soluble 
in  alcohol  and  ether.  It  is  only  a  feeble  base,  and  its  salts 
are  decomposed  by  water  with  separation  of  the  base  ;  its 
solution  in  concentrated  sulphuric  acid  gives  with  a  trace 
of  nitrous  acid  an  intense  blue  colouration,  and  it  therefore 
serves  as  a  very  delicate  test  for  nitrous  acid  or  nitrites. 
Diphenylamine  is  largely  used  in  the  manufacture  of  dyes, 
also  for  experiments  in  which  a  high  constant  temperature 
is  required,  as,  for  example,  in  determining  the  vapour 
density  of  substances  of  high  boiling-point  by  V.  Meyer's 
method.  When  treated  with  potassium,  diphenylamine  yields 
a  solid  potassium  derivative,  (C6H5)2NK,  the  presence  of  the 
two  phenyl-groups  being  sufficient  to  impart  to  the  >NH 
group  a  feeble  acid  character,  similar  to  that  of  imides  (part  i. 
p.  238). 

Triplienylamine,  (C6H5)3N,  may  be  prepared  by  heating 
potassium  diphenylamine  with  monobromobenzene  at  300°, 

(C6H6)2NK  +  C6H5Br  =  (C6H5)3N  +  KBr. 

It  is  a  colourless,  crystalline  substance,  melts  at  127°,  and 
has  no  basic  properties,  as  it  does  not  combine  even  with  the 
strongest  acids. 

Aromatic  Amines. 

The  true  aromatic  amines  —  namely,  those  compounds  in 
which  the  amido-group  is  united  with  carbon  of  the  side-chain, 
are  of  far  less  importance  than  the  amido-compounds,  and 
only  a  few  substances  of  this  class  have  been  thoroughly 
investigated. 

Benzylamine,  C6H5-CH2-NH2,  may,  however,  be  described 
as  a  typical  aromatic  primary  nmine.  It  may  be  obtained  by 


AMIDO-COMPOUNDS    AND    AMINES.  369 

reducing  phenyl  cyanide  (benzonitrile,  p.  421)  with  sodium 
and  alcohol, 

C6H5-CN  +  4H  =  C6H5-CH2.NH2, 

by   treating  the   amide   of  phenylacetic  acid   (p.    429)   with 
bromine  and  potash, 

C6H5.CH0-CO.NH2  +  Br,  +  4KOH  = 

C6H5.CH2-NH2  +  2KBr  +  K2C03  +  2H20, 

and  by  heating  benzyl  chloride  with  alcoholic  ammonia, 


C6H5-CH2C1  +  NH3  -  C6H5.CH2-NH2,  HC1. 

All   these   methods    are   similar  to  those  employed   in   the 
preparation  of  fatty  primary  amines. 

Benzylamine  is  a  colourless,  pungent  -smelling,  strongly 
basic  liquid,  boiling  at  185°;  it  closely  resembles  the  fatty 
amines  in  nearly  all  respects,  and  differs  from  the  monamido- 
compoimds  (aniline,  toluidine,  &c.)  in  being  readily  soluble  in 
water,  and  in  not  yielding  diazo-compounds  when  its  salts  are 
treated  with  nitrous  acid.  Like  the  fatty  primary  amines,  it 
gives  the  carbylamine  reaction,  and  is  converted  into  the 
corresponding  alcohol  (benzyl  alcohol,  p.  403)  on  treatment 
with  nitrous  acid. 

Secondary  and  tertiary  aromatic  amines  are  formed  when  a 
primary  amine  is  heated  with  an  aromatic  halogen  compound, 
containing  the  halogen  in  the  side-chain  ;  when,  for  example, 
benzylamine  is  heated  with  benzyl  chloride,  both  dibenzylamine 
and  tribenzylamine  are  produced,  just  as  diethylamine  and  triethyl- 
amine  are  obtained  when  ethylamine  is  heated  with  ethyl  bromide, 

C6H5.CH2-NH2  +  C6H5.CH2C1  =  (CeH^CH^NH,  HC1 
C6H5.CH2.NH2  +  2C6H5-CH2C1  =  (C6H5-CH2)3N,  HC1  +  HC1. 

When,  therefore,  benzyl  chloride  is  heated  with  ammonia,  the  pro- 
duct consists  of  a  mixture  of  the  salts  of  all  three  amines. 


370  DIAZO-COMPOUNDS   AND   THEIR   DERIVATIVES. 


CHAPTER     XXIV. 

DTAZO-COMPOUNDS    AND    THEIR   DERIVATIVES. 

It  has  already  been  stated  that  when  the  amido-compounds 
or  their  salts  are  treated  with  nitrous  acid  in  aqueous  solution, 
they  yield  phenols;  this  decomposition,  however,  usually 
takes  place  only  on  warming.  If,  for  example,  a  well-cooled 
dilute  solution  of  aniline  hydrochloride  (1  mol.)  be  mixed 
with  sodium  nitrite  (1  mol.),  and  hydrochloric  acid  (1  mol.) 
added  to  set  free  the  nitrous  acid,  phenol  is  not  pro- 
duced, and  the  solution  contains  a  very  unstable  substance 
called  diazobenzene  chloride,  the  formation  of  which  may  be 
expressed  by  the  equation 

C6H5.NH2,HC1  +  N02H  =  C6H5-N:NC1  +  2H20. 

In  this  respect,  then,  the  amido-compounds  differ  from  the 
fatty  amines ;  the  latter  are  at  once  converted  into  alcohols  by 
nitrous  acid  in  the  cold,  whereas  the  former  are  first  trans- 
formed into  diazo-compounds,  which,  usually  only  on  warming, 
decompose  more  or  less  readily  with  formation  of  phenols 
(p.  386). 

All  amido-compounds  behave  in  this  way,  yielding  diazo- 
salts  similarly  constituted  to  diazobenzene  chloride. 

The  diazo-salts  were  discovered  in  1860  by  P.  Griess;  they 
may  be  assumed  to  be  salts  of  diazobenzene,  C6H5-]S[:N-OH, 
and  its  homologues.  substances  which  it  has  not  been  found 
possible  to  isolate  in  a  pure  state  and  analyse  on  account  of 
their  unstable  nature. 

The  diazo-salts  (usually  spoken  of  as  the  diazo-compounds) 
may  nevertheless  be  isolated  without  much  difficulty,  although, 
as  a  matter  of  fact,  they  are  seldom  separated  from  their 
aqueous  solutions,  partly  because  of  their  explosive  character, 


DIAZO-COMPOUNDS    AND    THEIR    DERIVATIVES.  371 

partly  because  for  most  purposes  for  which  they  are  prepared 
this  operation  is  quite  unnecessary. 

•Preparation. — Anhydrous  diazo-salts  may  be  obtained  by 
treating  a  well-cooled  solution  of  an  amido-compound  in 
absolute  alcohol  with  amyl  nitrite  and  a  mineral  acid,  in 
absence  of  any  considerable  quantity  of  water, 

C6H5.NH2,HC1  +  C6HU.0-NO  = 

C6H5.N:NC1  +  C5Hn-OH  +  H20. 

Diazobenzene  sulphate,  C6H5-N:N-S04H,  for  example,  is  pre- 
pared by  dissolving  aniline  (15  parts)  in  absolute  alcohol  (10  parts), 
adding  concentrated  sulphuric  acid  (20  parts),  and  after  cooling 
in  a  freezing  mixture,  slowly  running  in  pure  amyl  nitrite  (20 
grams) ;  after  10-15  minutes  diazobenzene  sulphate  separates  in 
crystals,  which  are  washed  with  alcohol  and  ether,  and  dried  in 
the  air  at  ordinary  temperatures. 

Diazobenzene  chloride  and  diazobenzene  nitrate  may  be  obtained 
in  a  similar  manner,  employing  alcoholic  solutions  of  hydrogen 
chloride  and  of  nitric  acid  in  the  place  of  sulphuric  acid. 

Diazobenzene  nitrate,  C6H5-N:N-N03,  may  also  be  conveniently 
isolated  as  follows  :  Aniline  nitrate  is  suspended  in  a  small  quantity 
of  water,  and  the  liquid  saturated  with  nitrous  acid  (generated 
from  As2O3  and  HNO3),  when  the  crystals  gradually  dissolve  with 
formation  of  diazobenzene  nitrate  ;  on  the  addition  of  alcohol  and 
ether,  this  salt  separates  in  colourless  needles.  Special  precau- 
tions are  to  be  observed  in  preparing  this  substance,  as,  when  dry, 
it  is  highly  explosive,  although  it  may  be  handled  with  safety  if 
kept  moist. 

Aqueous  solutions  of  the  diazo-salts  are  prepared  by  dis- 
solving the  amido-compound  in  an  aqueous  mineral  acid,  and 
adding  the  theoretical  quantity  of  a  solution  of  sodium  nitrite, 
after  first  cooling  to  0°  (see  above,  also  p.  373). 

Properties. — The  diazo-salts  are  colourless,  crystalline  com- 
pounds, very  readily  soluble  in  water ;  in  the  dry  state  they 
are  more  or  less  explosive,  and  should  be  handled  only  with 
the  greatest  caution.  They  are  of  immense  value  in  syntheti- 
cal chemistry  and  in  the  preparation  of  dyes,  as  they  undergo 
a  number  of  remarkable  reactions,  of  which  the  following  are 
some  of  the  more  important, 


372  DIAZOCOMPOUNDS    AND   THEIR    DERIVATIVES. 

When  warmed  in  aqueous  solution  they  decompose  rapidly, 
with  evolution  of  nitrogen  and  formation  of  phenols  (p.  386), 

C6H5-N:N.N03  +  H20  -  C6H5.QH  +  N2  +  HN03 
C6H4(CH8).N:NC1  +  H20  -  C6H4(CH3)-OH  +  N2  +  HCL 

2>-Diazotoluene  Chloride.  p-Cresol. 

When  boiled  with  strong  alcohol  they  yield  hydrocarbons, 
part  of  the  alcohol  being  oxidised  to  aldehyde, 

C6H5.N:NC1  +  C2H5-OH  =  C6H6  +  N2  +  HC1  +  CH3-CHO. 

These  two  reactions  afford  a  means  of  obtaining  phenols 
and  hydrocarbons  from  amido-compounds. 

The  diazo-compounds  behave  in  a  very  remarkable  way 
when  treated  with  cuprous  salts ;  if,  for  example,  a  solution 
of  diazobenzene  chloride  be  warmed  with  cuprous  chloride, 
nitrogen  is  evolved,  and  chlorobenzene  is  produced.  In  this 
reaction,  the  diazo-salt  combines  with  the  cuprous  chloride 
to  form  an  intermediate  brownish  additive  compound,  which 
is  decomposed  at  higher  temperatures,  cuprous  chloride 
being  regenerated ;  theoretically,  therefore,  the  reaction  is 
continuous, 
Htlf  C6H6.N:NC1,  Cu2Cl2  =  C6H5C1  +  N2  +  Cu2Cl2. 

If,  instead  of  the  chloride,    cuprous  bromide  or  cuprous 
i^krdide  be  employed,  bromobenzene  or  iodobenzene  is  produced, 

C0H6.N:NBr,Cu2Br2  -  C6H5Br  +  N2  +  Cu2Br2, 

Additive  Compound.  Bromobenzene. 

whereas  by   using  cuprous  cyanide,   a   cyanide   or  nitrile  is 
formed, 

CflHg.Njj.CN,  Cu2(ON)2  =  C6H5.CN  +  N2  +  Cu2(CN)2. 

Additive  Compound.  Phenyl  Cyanide. 

In  this  latter  reaction  a  mixture  of  cupric  sulphate  and 
potassium  cyanide  is  generally  used  instead  of  the  previously 
prepared  cuprous  cyanide. 

By  means  of  this  very  important  reaction,  which  was 
discovered  by  Sandmeyer  in  1884,  it  is  possible  to  displace  the 


DIAZO-COMPOUNDS   AND   THEIR   DERIVATIVES.  373 

NH2-  group  in  amide-compounds  by  Cl,  Br,  I,  CN,  and 
indirectly  by  COOH  (by  the  hydrolysis  of  the  CN-  group),  and 
indeed  by  other  atoms  or  groups ;  as,  moreover,  the  yield  is 
generally  good,  Sandmeyer's  reaction  is  of  great  practical 
value.  The  amido-compounds  being  readily  obtainable  from  the 
nitro-compounds,  and  the  latter  from  the  hydrocarbons,  this 
method  affords  a  means  of  preparing  halogen,  cyanogen,  and 
other  derivatives  indirectly  from  the  hydrocarbons. 

Gattermann  has  shown  that  the  decomposition  of  the  diazo- 
compounds  is,  in  many  cases,  best  brought  about  by  treating  the 
cold  solution  of  the  diazo-salt  with  copper  powder  (prepared  by  the 
action  of  zinc-dust  on  a  solution  of  copper  sulphate).  Monochlor- 
benzene,  for  example,  is  readily  obtained  from  aniline  by  the 
following  process  :  Aniline  (31  grams)  is  dissolved  in  hydrochloric 
acid  (300  grams)  and  water  (150  grams),  the  solution  well  cooled 
with  ice,  and  diazotised  by  adding  gradually  a  concentrated  aqueous 
solution  of  sodium  nitrite  (23  grams).  The  solution  of  diazobenzene 
chloride  thus  obtained  is  gradually  mixed  with  copper  powder  (40 
grams),  when  nitrogen  is  evolved  and  chlorobenzene  produced,  the 
reaction  being  complete  in  about  half  an  hour.  The  chlorobenzene 
is  then  purified  by  distillation  in  steam  and  fractionation. 

In  preparing  cyanobenzene,  C6H5-CN,  from  aniline,  aniline  sul- 
phate is  diazotised,  the  solution  mixed  with  potassium  cyanide,  and 
then  copper  powder  added. 

The  diazo-com pounds  also  serve  for  the  preparation  of  an 
important  class  of  compounds  known  as  the  hydrazines,  these 
substances  being  obtained  by  reducing  the  diazo-compounds, 
usually  with  stannous  chloride  and  hydrochloric  acid, 

R.N:XC1  +  4H  =  R.NH.NH2,HC1. 

Diazochloride.  Hydrazine  Hydrochloride. 

Constitution  of  Diazo-compounds. — That  diazobenzene  salts 
have  the  constitution  expressed  by  the  formula  C6H5'N:NR 

6     a 

(where  R  =  Cl,  Br,  I,  N03,  HS04,  &c.)  is  shown  by  the 
following  considerations.  On  reduction  they  are  converted 
into  phenylhydrazine,  C6H5-NH-NH2  (the  constitution  of 
which  is  known,  p.  376),  a  fact  which  shows  that  the  two 
nitrogen  atoms  are  united  together,  and  that  one  of  them  (b) 


374          DIAZO-COM POUNDS  AND  THEIR  DERIVATIVES. 

is  combined  with  the  benzene  nucleus.     Diazobenzene  chloride 
interacts  readily  with  dimethylaniline,  giving  dimethylamido- 
azobenzene  (p.  376), 
C6H5.N:NC1  +  C6H6.N(CH3)2  = 

C6H5.N:N.C6H4.N(CH3)2  +  HC1, 

b    a 

and  this  substance,  on  reduction,  yields  aniline  and  dimethyl- 
^9-phenylenediamine  (p.  376), 
C6H5.N:N-C6H4.N(CH3)2  +  4H  = 

C6H5.NH2  +  NH2.C6H4.N(CH3)2. 

b  a 

These  changes  can  only  be  explained  on  the  assumption  that 
the  acid  radicle  is  attached  to  the  a-nitrogen  atom,  as  in  the 
above  formula,  because  if  it  were  united  to  the  other  nitrogen 
atom  (6),  as  in  the  formula  C6H5-NC1 :  N,  for  example,  such 

b  a 

products  could  not  be  obtained. 

Free  diazobenzene  is  very  unstable,  and  has  not  been  obtained  in 
a  pure  state,  but  it  probably  has  the  constitution  C6H5-N:N'OH. 

Diazoamido-  and  Amidoazo-compounds. 

Although  some  of  the  more  characteristic  reactions  of 
diazo-compounds  have  already  been  mentioned,  there  are 
numerous  other  changes  of  great  interest  and  of  great  com- 
mercial importance  which  these  substances  undergo. 

When,  for  example,  diazobenzene  chloride  is  treated  witli 
aniline,  a  reaction  takes  place  similar  to  that  which  occurs 
when  aniline  is  treated  with  benzoyl  chloride  (p.  420),  and 
diazoamidobenzene  is  formed, 

C6H5.N:NC1  +  NH2.CflH5  =  C6H5.N:N-NH.C6H5  +  HC1 

Diazoamidobenzene. 

C6H5.COC1  +  NH2.C6H6  =  C6H5.CO-NH.C6H5  +  TIC1. 

Benzoylaniidobenzene  or  Benzanilide. 

As,  moreover,  other  diazo-compounds  and  other  amido- 
compounds  interact  in  a  similar  manner,  numerous  diazoamido- 
compounds  may  be  obtained. 

Diazoamidobenzene,     C6H5-N:N-NH-C6H5,    may    be    de- 


DIAZOCOMPOUNDS    AND   THEIR   DERIVATIVES.  375 

scribed  as  a  typical  compound  of  this  class ;  it  is  conveniently 
prepared  by  passing  nitrous  fumes  into  an  alcoholic  solution 
of  aniline,  the  diazobenzene  nitrite,  which  is  probably  first 
produced,  interacting  with  excess  of  aniline, 

C6H5-N:]ST^T02  +  C6H5.NH2  - 

C6H5.N:X.NILC6H5  +  HN02. 

Diazoamidobenzene  crystallises  in  brilliant  yellow  needles, 
and  is  sparingly  soluble  in  water,  but  readily  in  alcohol  and 
ether ;  it  does  not  form  salts  with  acids. 

Amidoazobenzene,  C6H5-X:X-C6H4-NH2,  is  formed   when 
diazoamidobenzene  is  warmed  with  a  small  quantity  of  aniline 
hydrochloride  at  40°,  intramolecular  change  taking  place, 
C6H5.X:.\.XH.C6H5  =  C6H6'N:N.C6H4.NH2. 

The  course  of  this  remarkable  reaction,  which  is  a  general  one, 
and  shown  by  all  diazoamido-compounds,  may  possibly  be 
explained  by  assuming  that  the  aniline  hydrochloride  first  decom- 
poses the  diazoamidobenzene,  yielding  diazobenzene  chloride  and 
aniline  thus  : 
C6H5.N:N.NH.C6H5  +  C6H5-NH2,  HC1  -  C6H5.N:NC1  +  2C6HB-NH, 

The  diazobenzene  chloride  then  interacts  with  excess  of  aniline  in 
such  a  way  that  the  diazo-group  displaces  hydrogen  of  the  nucleus 
from  the  j9«7-a-position  to  the  amido-group, 

C6H5-N  :NC1  +  2CtiH5.NH2= C6H5-N  :N-C6H4.NH2  +  C6H5-NH2,  HC1. 

The  change  is,  therefore,  theoretically  continuous,  the  regenerated 
aniline  hydrochloride  being  able  to  convert  a  further  quantity  of 
the  diazoamidobenzene  into  the  amidoazo-compound. 

Amidoazobenzene  may  also  be  prepared  by  nitrating 
azobenzene  (p.  378),  and  then  reducing  the  ^-nitroazo- 
benzene,  C6H5-X:X-C6H4-X02,  which  is  produced  with 
ammonium  sulphide,  a  series  of  reactions  analogous  to  those 
which  occur  in  the  formation  of  aniline  from  benzene,  and 
which  prove  the  constitution  of  amidoazobenzene. 

Amidoazobenzene  crystallises  from  alcohol  in  brilliant 
orange-red  plates,  and  melts  at  125°;  its  salts  are  intensely 
coloured,  the  hydrochloride,  C6H5-X:X.C6H4.NH2,  HC1,  for 
example,  forms  beautiful  steel-blue  needles,  and  used  to  come 


376  DIAZO-COMPOUNDS    AND    THEIR    DERIVATIVES. 

into  the  market  under  the  name  of  '  aniline  yellow '  as  a  silk 
dye  (p.  524). 

Other  amidoazo-compounds  may  be  obtained  directly  by  treating 
tertiary  alkylanilines  (p.  364)  with  diazo-salts :  dimethylaniline,  for 
example,  interacts  with  diazobenzene  chloride,  yielding  dimethyl- 
amidoazobenzene, 

C6H5.N:NC1  +  C6H5.N(CH3)2  =  C6H5.N:N.C6H4.N(CH3)2,  HC1, 

no    intermediate    diazoamido-compound     being    formed,    because 
dimethylaniline  does  not  contain  an  NIK  or  NH2-  group. 

In  this  case  also  the  diazo-group,  C6H5-N:N-,  takes  up  the 
^-position  to  the  N(CH3)2-  group,  as  is  shown  by  the  fact  that, 
on  reduction,  dimethylamidoazobenzene  is  converted  into  aniline 
and  dimethyl-p-phenylenediamine,  the  latter  being  identical  with 
the  base  which  is  produced  by  reducing  ^-nitrosodimcthylaniliiie 
(p.  367). 

Phenylhydrazine,  C6H5.NH.NH2,  a  compound  of  great 
practical  importance,  is  easily  prepared  by  the  reduction  of 
diazobenzene  chloride, 

C6H5.N:NC1  +  4H  =  C6H6.NH-NH2,  HC1. 

Aniline  (10  grams)  is  dissolved  in  concentrated  hydrochloric  acid 
(200  c.c.),  and  to  the  well-cooled  solution  sodium  nitrite  (7 -5  grams) 
dissolved  in  water  (50  c.c.)  is  added  in  small  quantities  at  a  time; 
the  resulting  solution  of  diazobenzene  chloride  is  then  mixed  with 
stannous  chloride  (45  grams)  dissolved  in  concentrated  hydrochloric 
acid  (45  grams).  The  precipitate  of  phenylhydrazine  hydrochloride, 
which  rapidly  forms,  is  separated  by  nitration,  dissolved  in  water, 
decomposed  with  potash,  and  the  free  base  extracted  with  ether 
and  purified  by  fractionation. 

Phenylhydrazine  crystallises  in  colourless  prisms,  melts  at 
23°,  and  boils  with  slight  decomposition  at  241°,  so  that  it  is 
best  purified  by  distillation  under  reduced  pressure.  It  is 
sparingly  soluble  in  cold  water,  readily  in  alcohol  and  ether ; 
it  is  a  strong  base,  and  forms  well-characterised  salts,  such  as 
the  hydrochloride,  C6H5-NH-NH2,  HC1,  which  crystallises  in 
colourless  needles,  and  is  readily  soluble  in  hot  water ; 
solutions  of  the  free  base  and  of  its  salts  reduce  Fehling's 
solution  in  the  cold.  The  constitution  of  phenylhydrazine  is 
established  by  the  fact  that,  when  heated  with  zinc-dust 


DIAZO-COMPOUNDS    AND   THEIR    DERIVATIVES.  377 

and    hydrochloric    acid,    it    is    converted    into   aniline   and 
ammonia. 

Phenylhydrazine  interacts  readily  with  aldehydes,  ketones, 
and  other  substances  containing  a  carbonyl-group,  with  elimi- 
nation of  water  and  formation  of  plienylhydrazones  (hydra- 
zones)  ;  as  these  compounds  are  usually  sparingly  soluble  and 
often  crystallise  well,  they  may  frequently  be  employed  with 
advantage  in  the  identification  and  isolation  of  aldehydes, 
ketones,  &c.  (part  i.  p.  133), 


C6H5.CHO  +  C6H5-NH-NH2  =  C6H5.CH:N.NH-C6H5  +  H20 

Benzaldehyde.  Benzaldehyde  Hydrazone. 

C6H5.CO-CH3  +  C6H5.XH.NH2  = 

Acetophenone. 


+  HaO. 


Acetophenone  Hydrazone. 

Most  hydrazones  are  decomposed  by  strong  mineral  acids, 
with  regeneration  of  the  aldehyde  or  ketone,  and  formation  of 
a  salt  of  phenylhydrazine, 
C6H5-CH:X.^H  C6H5  +  H20  +  HC1  - 

C6H5-CHO  +  CeHg.NH-NHjp  HC1. 

The  value  of  phenylhydrazine  as  a  means  of  detecting  and 
isolating  the  sugars  has  been  explained  (part  i.  p.  267). 

In  preparing  hydrazones,  the  reacting  substances  may  either  be 
heated  together  without  a  solvent,  or  more  frequently  the  substance 
is  dissolved  in  water  (or  alcohol),  and  the  solution  of  the  requisite 
amount  of  phenylhyd  razine  in  dilute  acetic  acid  added.  On  warming, 
the  hydrazone  generally  separates  in  a  crystalline  form,  and  may  be 
readily  purified  by  recrystallisation. 

Osazones  (part  i.  p.  268)  are  prepared  by  warming  an  aqueous 
solution  of  a  sugar,  with  a  large  excess  of  phenylhydrazine  dissolved 
in  dilute  acetic  acid  ;  after  some  time  the  osazone  begins  to  be 
deposited  in  a  crystalline  form,  the  separation  increasing  as  the 
liquid  cools. 

Azo-compounds. 

It  has  already  been  shown  that  when  nitro-com  pounds  are 
treated  with  tin  and  hydrochloric  acid,  and  other  acid  reduc- 


378  DIAZO-COMPOUNDS    AND   THEIR   DERIVATIVES. 

ing  agents,  they  are  converted  into  amido-compounds,  a 
similar  change  taking  place  when  alcoholic  ammonium 
sulphide  is  employed;  when,  however,  nitro-compounds  are 
treated  with  other  alkaline  reducing  agents,  such  as  sodium 
amalgam,  staunous  oxide  and  soda,  or  zinc-dust  and  soda, 
they  yield  azo-compounds,  such  as  azobenzene,  two  molecules 
of  the  nitro-compound  affording  one  molecule  of  the  azo- 
compound, 

2C6H5.N02  +  4H  -  C6H5.N:N.C6H5  +  2H20. 

Azobenzene,  C6H5-N:N«C6H5,  may  be  described  as  a 
typical  example  of  this  class  of  compounds.  It  is  prepared 
by  agitating  nitrobenzene  with  the  calculated  quantity  of 
stannous  chloride,  dissolved  in  soda,  until  the  odour  of  nitro- 
benzene is  imperceptible.  The  reddish  precipitate  is  collected, 
washed  with  water,  dried,  and  recrystallised  from  light 
petroleum. 

Azobenzene  crystallises  in  brilliant  red  plates,  melts  at  68°, 
and  distils  at  293°;  it  is  readily  soluble  in  ether  and  alcohol, 
but  insoluble  in  water.  Alkaline  reducing  agents,  such  as 
ammonium  sulphide,  zinc-dust  and  soda,  &c.,  convert 
azobenzene  into  hydrazobenzene,  a  colourless,  crystalline  sub- 
stance, which  melts  at  131°, 

C6H5.N:N.C6H5  +  2H  =  C6H5.NH.NH.C6H6, 

whereas  a  mixture  of  zinc-dust  and  acetic  acid  decomposes  it, 
with  formation  of  aniline, 

C6H5.N:N.C6H6  +  4H  =  2C6H6.NH2. 
Other  azo-compounds  behave  in  a  similar  manner. 

Hydrazobenzene,  C6H5-NH-NH-C6H5,  is  readily  converted  into 
azobenzene  by  mild  oxidising  agents  such  as  mercuric  oxide,  and 
slowly  even  when  air  is  passed  through  its  alcoholic  solution. 
When  treated  with  strong  acids,  it  undergoes  a  very  remarkable 
intramolecular  change,  and  is  converted  into  jo-diamidodiphenyl  or 
benzidine,  a  strongly  basic  substance  largely  used  in  the  prepara- 
tion of  azo-dyes  (p.  526), 

C6H5.NH-NH.C6H5  =  NH2-C6H4.C6H4.NH2. 

Benzidine. 


DtA20COMPOUNDS    AND   THEIR    DERIVATIVES.  379 

Benzidine  may  be  directly  produced  by  reducing  azobenzene  with 
tin  and  strong  hydrochloric  acid  ;  other  azo-compounds,  such 
as  azo-toluene,  CH3-C6H4-N:N-C6H4-CH3,  behave  in  a  similar 
manner,  and  are  readily  converted  into  isomeric  alkyl-derivatives 
of  benzidine,  such  as  dimethylbenzidine  (tolidine), 


CHAPTER     XXV. 

SULPHONIC   ACIDS    AND    THEIR   DERIVATIVES. 

When  benzene  is  heated  with  concentrated  sulphuric  acid, 
it  gradually  dissolves,  and  benzenes  ulphonic  acid  is  formed  by 
the  substitution  of  the  sulphonic  group  -S03H  or  -S02-OH 
for  an  atom  of  hydrogen, 

C6H6  +  H2S04  =  C6H5-S03H  +  H20. 

The  homologues  of  benzene  and  aromatic  compounds  in 
general  behave  in  a  similar  manner,  and  this  property  of 
readily  yielding  sulphonic  derivatives  by  the  displacement  of 
hydrogen  of  the  nucleus  is  one  of  the  important  characteristics 
of  aromatic,  as  distinct  from  fatty,  compounds. 

The  sulphonic  acids  are  not  analogous  to  the  alkylsulphuric 
acids  (part  i.  p.  182),  which  are  ethereal  salts,  but  rather  to 
the  carboxylic  acids,  since  they  may  be  regarded  as  derived 
from  sulphuric  acid,  S02(OH)2,  just  as  the  carboxylic  acids 
are  derived  from  carbonic  acid,  CO(OH)2,  namely,  by  the 
substitution  of  an  aromatic  radicle  for  one  of  the  hydroxyl- 
groups. 

Sulphuric  acid,    S02<Oy        Carbonic    acid,    CO\/^TT 


Sulphonic  acid,    SOo^OH      Carboxylic  acid, 
Preparation. — Sulphonic  acids  are  prepared  by  treating  an 


380  SULPHONIC    ACIDS    AND    THEIR    DERIVATIVES, 

aromatic  compound  with  sulphuric  acid,  or  with  anhydrosul- 
phuric  acid, 

C6H6.CH3  +  H2S04  =  C6H4<go  H  +  H*° 


C6H6.NH2  +  H2S04  =  C6H4Jj  +  H20 
C6H6  +  2H2S04  =  C6H4<so3H  +  2H»a 

3 

The  number  of  hydrogen  atoms  displaced  by  sulphonic 
groups  depends  (as  in  the  case  of  nitre-groups)  on  the  tem- 
perature, on  the  concentration  of  the  acid,  and  on  the  nature 
of  the  substance  undergoing  sulplwnation. 

The  substance  to  be  sulphonated  is  mixed  with,  or  dissolved  in, 
excess  of  the  acid,  and,  if  necessary,  the  mixture  or  solution  is  then 
heated  on  a  water-  or  sand-bath  until  the  desired  change  is 
complete.  After  cooling,  the  product  is  carefully  treated  with 
water,  and  the  acid  isolated  as  described  later  (p.  382).  In  the  case  of 
substances  which  are  insoluble  in  water  or  dilute  sulphuric  acid,  the 
point  at  which  the  whole  is  converted  into  a  monosul  phonic  acid  is 
easily  ascertained  by  taking  out  a  small  portion  of  the  mixture  and 
adding  water  ;  unless  the  whole  is  soluble,  unchanged  substance  is 
still  present. 

Sometimes  chlorosulphonic  acid,  C1-SO3H,  is  employed  in  sul- 
phonating  substances,  and,  in  such  cases,  chloroform  or  carbon 
bisulphide  may  be  used  as  a  solvent  to  moderate  the  action, 

C6H6  +  C1.SO3H  =  C6H5.SO3H  +  HC1. 

Properties.  —  Sulphonic  acids  are,  as  a  rule,  colourless, 
crystalline  compounds,  readily  soluble  in  water,  and  often 
very  hygroscopic  ;  they  have  seldom  a  definite  melting-point, 
and  gradually  decompose  when  heated,  without  volatilising,  for 
which  reason  they  cannot  be  distilled.  They  have  a  sour 
taste,  a  strongly  acid  reaction,  turn  blue  litmus  red,  and  show, 
in  fact,  all  the  properties  of  powerful  acids,  their  basicity 
depending  on  the  number  of  sulphonic  groups  which  they 
contain.  They  decompose  carbonates,  and  dissolve  certain 
metals  with  evolution  of  hydrogen  ;  their  salts,  as  a  rule,  are 
readily  soluble  in  water. 

Although,  generally  speaking,  the  sulphonic  acids  are  very 


SULPHONIC   ACIDS    AND    THEIR    DERIVATIVES.  381 

stable,  and  are  not  decomposed  by  boiling  aqueous  alkalies  or 
mineral  acids,  they  undergo  certain  changes  of  great  import- 
ance. When  fused  with  potash  they  yield  phenols  (p.  387), 
and  when  strongly  heated  with  potassium  cyanide,  or  with 
potassium  ferrocyanide,  they  are  converted  into  cyanides  (or 
nitriles,  p.  421),  which  pass  off  in  vapour,  leaving  a  residue 
of  potassium  sulphite, 


C6H5.S03K  +  KCN  =  C6H5-CN  +  K2S03. 

The  sulphonic  group  may  also  be  displaced  by  hydrogen. 
This  may  be  done  by  strongly  heating  the  acids  alone,  or 
with  hydrochloric  acid  in  sealed  tubes,  or  by  passing  super- 
heated steam  into  the  acids,  or  into  their  solution  in  concen- 
trated sulphuric  acid. 

Sulphonic  acids  yield  numerous  derivatives,  which  may 
generally  be  prepared  by  methods  similar  to  those  used  in  the 
case  of  the  corresponding  derivatives  of  carboxylic  acids. 
When,  for  example,  a  sulphonic  acid  (or  its  alkali  salt)  is 
treated  with  phosphorus  pentachloride,  the  hydroxyl-group  is 
displaced  by  chlorine,  and  a  sulphonic  chloride  is  obtained, 

C6H5.S02-OH  +  PC15  =  C6H5.S02C1  +  POC13  +  HC1. 

All  sulphonic  acids  behave  in  this  way,  and  their  sulphonic 
chlorides  are  of  great  value,  not  only  because  they  are  often 
useful  in  isolating  and  identifying  the  ill-characterised  acids, 
but  also  because,  like  the  chlorides  of  the  carboxylic  acids, 
they  interact  readily  with  many  other  compounds. 

The  sulphonic  chlorides  are  decomposed  by  water  and  by 
alkalies,  giving  the  sulphonic  acids  or  their  salts  ;  they 
interact  with  alcohols,  yielding  ethereal  salts,  such  as  ethyl 
benzenesulph  onate, 

C6H5-S02C1  +  C2H5-OH  =  CflH5.S02-OC2H5  +  HC1, 
and  when  shaken  with  concentrated  ammonia  they  are  usually 
converted  into  well-defined  crystalline  sulphonamides,  which 
also  serve  for  the  identification  of  the  acids, 

C6H5.S02C1  +  NH3  =  C6H5.S02-NH2  +  HC1. 

Benzenesulphonic  Chloride.  Benzenesulphonamide. 


382  SULPHONIC    ACIDS    AND    THEIR    DERIVATIVES. 

The  isolation  of  sulphonic  acids  is  very  often  a  matter  of  some 
difficulty,  because,  like  the  sugars,  they  are  readily  soluble  in 
water  and  non-volatile,  and  cannot  be  extracted  from  their  aqueous 
solutions  by  shaking  with  ether,  &c.,  or  separated  from  other 
substances  by  steam  distillation.  The  first  step  usually  consists 
in  separating  them  from  the  excess  of  sulphuric  acid  employed  in 
their  preparation  ;  this  may  be  done  in  the  following  manner  : 
The  aqueous  solution  of  the  product  of  sulphonation  (see  above)  is 
boiled  with  excess  of  barium  (or  calcium)  carbonate,  filtered  from 
the  precipitated  barium  (or  calcium)  sulphate,  and  the  filtrate— 
which  contains  the  barium  (or  calcium)  salt  of  the  sulphonic  acid — 
treated  with  sulphuric  acid  drop  by  drop  as  long  as  a  precipitate  is 
produced  ;  after  again  filtering,  an  aqueous  solution  of  the  sulphonic 
acid  is  obtained,  and  on  evaporating  to  dryness,  the  acid  remains 
as  a  syrup  or  in  a  crystalline  form.  If  calcium  carbonate  has  been 
used,  the  acid  will  contain  a  little  calcium  sulphate,  which  may 
be  got  rid  of  by  adding  a  little  alcohol,  filtering,  and  again 
evaporating. 

Lead  carbonate  is  sometimes  employed  instead  of  barium  or 
calcium  carbonate ;  in  such  cases,  the  filtrate  from  the  lead  sul- 
phate is  treated  with  hydrogen  sulphide,  filtered  from  lead  sulphide, 
and  then  evaporated.  These  methods  are,  of  course,  only  applic- 
able provided  that  the  barium,  calcium,  or  lead  salt  of  the  acid  is 
soluble  in  water ;  in  other  cases  the  separation  is  much  more 
troublesome. 

When  two  or  more  sulphonic  acids  are  present  in  the  product, 
they  are  usually  separated  by  fractional  crystallisation  of  their 
salts,  after  first  getting  rid  of  the  sulphuric  acid  as  just  described  ; 
the  alkali  salts  are  easily  prepared  from  the  barium,  calcium,  or 
lead  salts  by  treating  the  solution  of  the  latter  with  the  alkali 
carbonate  as  long  as  a  precipitate  is  produced,  filtering  from  the 
insoluble  carbonate,  and  then  evaporating. 

Sometimes  a  complete  separation  cannot  be  accomplished  with 
the  aid  of  any  of  the  salts,  or  the  salts  and  the  acids  themselves 
are  so  badly  characterised  that  it  is  difficult  to  make  sure  of  their 
purity;  in  such  cases  the  sulphonic  chlorides  are  prepared  by 
treating  the  alkali  salts  with  phosphorus  pentachloride ;  these 
compounds  are  soluble  in  ether,  chloroform,  &c.,  and  generally 
crystallise  well,  so  that  they  are  easily  separated  and  obtained  in 
a  state  of  purity. 

Benzenesulphonic  acid,  C6H5-S03H,  is  prepared  by  gently 
boiling  a  mixture  of  equal  volumes  of  benzene  and  con- 


SULPHONIC    ACIDS    AND    THEIR    DERIVATIVES.  383 

centrated  sulphuric  acid  for  twenty  to  thirty  hours,  using 
a  reflux  condenser ;  it  is  isolated  with  the  aid  of  its  barium 
or  lead  salt,  both  of  which  are  soluble  in  water.  It 
crystallises  with  1|  mols.  H.20  in  colourless,  hygroscopic 
plates,  and  dissolves  freely  in  alcohol ;  when  fused  with 
potash,  it  yields  phenol  (p.  391).  Benzenesulphonic  chloride, 
C6H5-S02C1,  is  an  oil,  but  the  sulphonamide,  C6H5-S02-NH2, 
is  crystalline,  and  melts  at  149°. 

Benzene- w-disulphonic  acid,  C6H4(S03H)2,  is  also  pre- 
pared by  heating  the  hydrocarbon  with  concentrated  sulphuric 
acid,  but  a  larger  proportion  (two  volumes)  of  the  acid  is 
employed,  and  the  solution  is  heated  more  strongly  (or 
anhydrosulphuric  acid  is  used) ;  it  may  be  isolated  by  means 
of  its  barium  salt,  and  thus  obtained  in  crystals  containing 
2J  mols.  H20,  but  it  is  very  hygroscopic.  When  fused  with 
potash,  it  yields  resorcinol  (p.  398). 

Benzene-o-disulphonic  add  and  the  corresponding  ^-com- 
pound are  of  little  importance. 

The  three  (o.m.p.)  toluenesulphomc  acids,  C6H4(CH3)-S03H, 
are  crystalline,  and  their  barium  salts  are  soluble  in  water; 
only  the  o-  and  ^5-acids  are  formed  when  toluene  is  dissolved 
in  aQhydrosulphuric  acid. 

Sulphanilic  acid,  amidobenzene-j>sulphonic  acid,  or  aniline- 
£>-sulphonic  acid,  C6H4(NH2)-S03H,  is  easily  prepared  by 
heating  aniline  sulphate  at  about  200°  for  some  time. 

Aniline  is  slowly  added  to  a  slight  excess  of  the  theoretical 
quantity  of  sulphuric  acid  contained  in  a  porcelain  dish,  the 
mixture  being  constantly  stirred  as  it  becomes  solid  ;  the  dish  is 
then  gently  heated  on  a  sand-bath,  the  contents  being  stirred,  and 
care  being  taken  to  prevent  charring.  The  process  is  at  an  end 
as  soon  as  a  small  portion  of  the  product,  dissolved  in  water,  gives 
no  oily  precipitate  of  aniline  on  adding  excess  of  soda.  After 
cooling,  a  little  water  is  added,  the  sparingly  soluble  sul phonic 
acid  separated  by  filtration,  and  purified  by  recry stall isation  from 
boiling  water,  with  addition  of  animal  charcoal  (see  foot-note,  p.  393). 

Sulphanilic  acid  crystallises  with  2  mols.  H00,  and  is 
readily  soluble  in  hot,  but  only  sparingly  in  cold,  water. 


384  SULPHONIC    ACIDS    AND    THEIR    DERIVATIVES. 

It  forms  salts  with  bases,  but  it  does  not  combine  with 
acids,  the  basic  character  of  the  amido-group  being  neutral- 
ised by  the  acid  character  of  the  sulphonic  group;  in  this 
respect,  therefore,  it  differs  from  glycine  (part  i.  p.  292), 
which  forms  salts  both  with  acids  and  bases. 

When  sulphanilic  acid  is  dissolved  in  dilute  soda,  the 
solution  mixed  with  a  slight  excess  of  sodium  nitrite,  and 
poured  into  well-cooled,  dilute  sulphuric  acid,  diazobenzene- 
sulphonic acid  is  formed, 

P  TT  XNH2        ,     TTH  ATr>          P  TT  ^NlN-OH 
°6H4<xS08H    4  C6H4\S03H  +    H2°; 

this  compound,  however,  immediately  loses  water,  and  is 
converted  into  its  anhydride,*  C6H4<^gQ  J^,  which  separ- 

ates from  the  solution  in  colourless  crystals. 

Diazobenzenesulphonic  acid,  or  rather  its  anhydride,  shows 
the  characteristic  properties  of  diazo-compounds  in  general  ; 
when  boiled  with  water,  it  is  converted  into  phenol-^-sul- 
phonic  acid  (p.  395), 


H 

o  3 


whereas,  when  heated  with  concentrated  hydrochloric  or 
hydrobromic  acid,  it  gives  chlorobenzene-  or  bromobenzene-^?- 
sulphonic  acid,t 


HC1  =  ^H4<o.H  +  N2- 

3  3 

Amidobenzene-o-sulphonic  acid  and  the  m-acid  (metanilic  acid) 
may  be  obtained  by  reducing  the  corresponding  nitrobenzene- 
sulphonic  acids,  C6H4(NO2)-S03H,  botli  of  which  are  formed,  to- 
gether with  the  jt>-acid,  on  nitrating  benzenesulphonic  acid  ;  they 

*  The  existence  of  this  anhydride  (and  of  that  of  amidobenzene-m- 
sulphonic  acid),  is  a  very  interesting  fact,  because,  as  a  rule,  anhydride 
formation  takes  place  only  between  groups  in  the  o-position  to  one 
another  (compare  p.  424). 

t  Many  other  diazo-compounds  which,  like  diazobenzenesulphonic  acid, 
contain  some  acid  group,  are  decomposed  by  halogen  acids  in  a  similar 
manner. 


SULPHONIC    ACIDS    AND    THEIR    DERIVATIVES.  385 

resemble  sulphanilic  acid  in  properties,  and  are  readily  converted 
into  the  anhydrides  of  the  corresponding  diazobenzenesulphonic 
acids. 

Many  other  sulphonic  acids  are  described  later. 


CHAPTER    XXVI. 

PHENOLS. 

The  hydroxy-com pounds  of  the  aromatic  series,  such  as 
phenol  or  hydroxy-benzene,  C6H5-OH,  the  isomeric  hydroxy- 
toluenes,  C6H4(CH3)-OH,  and  benzyl  alcohol,  C6H5.CH2-OH, 
are  theoretically  derived  from  the  aromatic  hydrocarbons  by 
the  substitution  of  hydroxyl-groups  for  atoms  of  hydrogen, 
just  as  the  fatty  alcohols  are  derived  from  the  paraffins.  It 
will  be  seen,  however,  from  the  examples  just  given  that 
whereas,  in  benzene,  hydrogen  atoms  of  the  nucleus  must 
necessarily  be  displaced,  in  the  case  of  toluene  and  all  the 
higher  homologues  this  is  not  so,  since  the  hydroxyl-groups 
may  displace  hydrogen  either  of  the  nucleus  or  of  the  side- 
chain.  Now  the  hydroxy-derivatives  of  benzene,  and  all 
those  aromatic  hydroxy-compounds,  formed  by  the  substitu- 
tion of  hydroxyl-groups  for  hydrogen  atoms  of  the  nucleus, 
differ  in  many  respects  not  only  from  the  fatty  alcohols,  but 
also  from  those  aromatic  compounds  which  contain  the 
hydroxyl-group  in  the  side-chain ;  it  is  convenient,  therefore, 
to  make  some  distinction  between  the  two  kinds  of  aromatic 
hydroxy-compounds,  and  for  this  reason  they  are  classed  in 
two  groups,  (a)  the  phenols,  and  (b)  the  aromatic  alcohols 
(p.  402). 

The  phenols,  then,  are  hydroxy-compounds  in  which  the 
hydroxyl-groups  are  united  directly  with  carbon  of  the 
nucleus ;  they  may  be  subdivided  into  monohydric,  dihydric, 
trihydric  phenols,  &c.,  according  to  the  number  of  hydroxyl- 
groups  which  they  contain.  Phenol,  or  carbolic  acid, 
CrH-;OH,  for  example,  is  a  monohydric  phenol,  as  are  also 

Y 


386  PHENOLS. 

the  three  isomeric  cresols  or  hydroxy toluenes,  C6H4(CH3)-OH; 
the  three  isomeric  dihydroxybenzenes,  C6H4(OH)2,  on  the 
other  hand,  are  dihydric  phenols,  whereas  phloroglucinol, 
C6H3(OH)3,  is  an  example  of  a  trihydric  compound. 

Many  of  the  phenols  are  easily  obtainable,  well-known 
compounds ;  carbolic  acid,  for  instance,  is  prepared  from 
coal-tar  in  large  quantities ;  carvacrol  and  thymol  occur  in 
various  plants,  and  catechol,  pyrogallol,  &c.,  may  be  obtained 
by  the  dry  distillation  of  certain  vegetable  products. 

Preparation. — Phenols  may  be  prepared  by  treating  salts 
of  amido-compounds  with  nitrous  acid  in  aqueous  solution, 
and  then  heating  until  nitrogen  ceases  to  be  evolved, 

C6H5.NH2,HC1  +  HO-NO  -  C6H5-OH  +  N2  +  H20  +  HC1 
C6H4<™3 1  HC1  +  HO.NO  =  CflH4<^3  +  N2  +  H20  +  HC1. 

It  is  possible,  therefore,  to  prepare  phenols,  not  only  from 
the  amido-compounds  themselves,  but  also  indirectly  from 
the  corresponding  nitro-derivatives  and  from  the  hydro- 
carbons, since  these  substances  may  be  converted  into  amido- 
compounds, 

Benzene.  Nitrobenzene.  Amidobenzene.  Phenol. 

C6H6  C6H5.N02  C6H5.NH2  C6H5.QH. 

The  conversion  of  an  amido-com pound  into  a  phenol  really 
takes  place  in  two  stages,  as  already  explained  (p.  370) ;  at 
ordinary  temperatures  the  salt  of  the  amido-compound  is 
transformed  into  a  salt  of  a  diazo-compound,  but  on  heating 
its  aqueous  solution,  the  latter  decomposes,  yielding  a  phenol, 

C6H5.NH2,  HC1  +  HC1  +  KN02  -  C6H5-N:NC1  +  KC1  +  211,0 
C6H5-N:NC1  +  H20  =  C6H5-OH  +  HC1  +  N2. 

The  amido-compound,  aniline,  for  example,  is  dissolved  in 
moderately  dilute  hydrochloric  acid  (2  mols.),  or  sulphuric  acid 
(1  mol.),  the  solution  is  cooled  in  ice  or  water,  and  an  aqueous 
solution  of  sodium  nitrite  (1  mol.)  is  slowly  added,  stirring  con- 
stantly. The  mixture  is  then  gradually  heated  to  boiling  on  a 
reflux  condenser,  until  the  evolution  of  nitrogen  (which  at  first 
causes  brisk  effervescence)  is  at  an  end,  and  the  diazo-salt  is  com- 


PHENOLS.  387 

pletely  decomposed  ;  the  phenol  is  afterwards  separated  from  the 
tarry  matter,  which  is  almost  invariably  produced,  either  by  dis- 
tillation in  steam,  by  crystallisation  from  hot  water,  or  by  extrac- 
tion with  ether;  in  the  last  case  the  ethereal  solution  is  usually 
shaken  with  soda,  which  dissolves  out  the  phenol,  leaving  most  of 
the  impurities  in  the  ether. 

Dihydric  phenols  may  sometimes  be  prepared  from  the 
corresponding  di-substitution  products  of  the  hydrocarbon,  as 
indicated  by  the  following  series  of  changes  : 

Benzene.    Dinitrobenzene.      Diamidobenzene.         Diazo-salt.         Dihydric  Phenol. 

CTJ     n  TI  /^^    n  TT  x'^'-H-o    r«  TI  S^f^    n  TJ 
6M6    U6±i4\]sr02       6    4  >NH2       6    4  >N2C1       6    4 

They  may  also  be  obtained  from  the  monohydric  compounds 
in  the  following  manner  : 

Phenol.          Nitrophenol.       Amidophenol.  Diazo-salt.         Dihydric  Phenol. 

CH.OH  CH<™2  C 


These  two  methods,  however,  are  limited  in  their  application, 
because  o-  and  ??j-diamido-com  pounds  cannot  always  be  con- 
verted into  the  corresponding  diazo-salts,  but  more  often  yield 
products  of  quite  a  different  nature  ;  o-  and  p-amido-hydroxy- 
compounds  also  show  an  abnormal  behaviour  with  nitrous 
acid,  the  former  not  being  acted  on  at  all,  the  latter  only 
with  difficulty.  For  these  reasons  dihydric  phenols  are 
usually  most  conveniently  prepared  by  the  methods  given 
later. 

Another  important  general  method  of  preparing  phenols 
consists  in  fusing  sulphonic  acids  or  their  salts  with  potash 
or  soda  ;  in  this  case,  also,  their  preparation  from  the  hydro- 
carbons is  often  easily  accomplished,  since  the  latter  are 
usually  converted  into  sulphonic  acids  without  difficulty, 

C6H5-S03K  +  KOH  =  C6H5.OH*  +  K2S03 


The  sulphonic  acid  or  its  alkali  salt  is  placed  in  an  iron,  or,  better, 
*  In  all  cases  the  phenols  are  present  in  the  product  as  alkali  salts. 


388  PHENOLS. 

nickel  or  silver  dish,*  together  with  excess  of  solid  potash  (or  soda), 
and  a  little  water,  and  the  dish  is  heated  over  a  free  flame,  the 
mixture  being  constantly  stirred  with  a  nickel  or  silver  spatula,  or 
with  a  thermometer,  the  bulb  of  which  is  encased  in  a  glass  tube, 
or  covered  with  silver  by  electro-deposition ;  after  the  potash  and 
the  salt  have  dissolved,  the  temperature  is  slowly  raised,  during 
which  process  the  mixture  usually  undergoes  a  variety  of  changes 
in  colour,  by  which  an  experienced  operator  can  tell  when  the 
decomposition  of  the  sulphonic  acid  is  complete ;  as  a  rule,  a 
temperature  considerably  above  200°  is  required,  so  that  simply 
boiling  the  sulphonic  acid  with  concentrated  potash  does  not  bring 
about  the  desired  change.  When  the  operation  is  finished,  the 
fused  mass  is  allowed  to  cool,  dissolved  in  water,  the  solution 
acidified  with  dilute  sulphuric  acid,  and  the  liberated  phenol  ex- 
tracted with  ether,  or  isolated  in  some  other  manner. 

Dihydric  phenols  may  often  be  obtained  in  a  similar  manner 
from  the  disulphonic  acids, 

C6H4(S03K)2  +  2KOH  =  C6H4(OH)2  +  2K2S03. 

Owing  to  the  high  temperature  at  which  these  reactions  must 
be  carried  out,  secondary  changes  very  often  occur.  When 
the  sulphonic  acid  contains  halogen  atoms,  the  latter  are 
usually  displaced  by  hydroxyl-groups,  especially  if  other  acid 
radicles,  such  as  -N02,  or  -S03H,  are  also  present;  when, 
for  example,  chlorobenzenesulphonic  acid,  C6H4C1-S03H,  is 
fused  with  potash,  a  dihydric  phenol,  C6H4(OH)2,  is  produced, 
the  halogen  as  well  as  the  sulphonic  group  being  eliminated. 
For  this  reason  also,  .compounds  such  as  o-  and  ^-chloro- 
nitrobenzene  may  be  converted  into  the  corresponding  nitro- 
phenols  (p.  392),  even  by  boiling  them  with  concentrated 
potash,  the  presence  of  the  nitro-group  facilitating  the  dis- 
placement of  the  halogen  atom ;  m-chloronitrobenzene,  on 
the  other  hand,  is  not  acted  on  under  these  conditions.  Some- 
times also  the  process  is  not  one  of  direct  substitution  only 
—that  is  to  say,  the  hydroxyl-groups  in  the  product  are  not 
united  with  the  same  carbon  atoms  as  those  with  which  the 
displaced  atoms  or  groups  were  united;  the  three  (o.m.p.) 

*  Caustic  alkalies  readily  attack  platinum  and  porcelain  at  high  tempera- 
tures, but  have  little  action  on  nickel  and  none  on  silver. 


PHENOLS.  389 

broinobehzenesulphonic  acids,  for  example,  all  yield  one  and 
the  same  dihydric  phenol  —  namely,  the  w-compound,  resor- 
cinol,  C6H4(OH)2,  because  the  o-  and  ^-dihydric  compounds, 
which  are  first  produced  from  the  corresponding  bromo- 
sulphonic  acids,  are  converted  into  the  more  stable  ?/i-deriva- 
tive  by  intramolecular  change. 

There  are  several  other  less  important  methods  by  which  phenols 
may  he  obtained,  as,  for  example,  by  distilling  hydroxy-acids, 
such  as  salicylic  acid,  with  lime, 

=  C6H5-OH  +  CO,, 

a  reaction  which  is  similar  to  that  which  occurs  in  preparing  the 
hydrocarbons  from  the  acids. 

Also  by  heating  other  phenols  with  fatty  alcohols  in  presence  of 
zinc  chloride,  when  the  alkyl-group  displaces  hydrogen  of  the 
nucleus,  just  as  in  the  production  of  toluidine,  &c.,  from  aniline 
(p.  357), 

C6H5.OH  +  C2H5-OH  -  C6H4<A  +  H2O. 


Properties.  —  Most  phenols  are  colourless,  crystalline  sub- 
stances, readily  soluble  in  alcohol  and  ether  \  their  solubility 
in  water  usually  increases  with  the  number  of  hydroxyl- 
groups  in  the  molecules,  phenol  and  cresol,  for  example, 
being  sparingly  soluble,  whereas  the  three  dihydric  phenols 
and  the  trihydric  compounds  are  readily  soluble.  Conversely, 
their  volatility  diminishes,  so  that  although  phenol  and  cresol 
distil  without  decomposition,  and  are  readily  volatile  in  steam, 
the  trihydric  phenols  usually  undergo  decomposition,  and 
volatilise  very  slowly  in  steam.  Alcoholic  and  aqueous 
solutions  of  phenols  (and  of  their  carboxylic  acids)  give  a 
violet,  blue,  or  green  colouration  with  ferric  salts,  the  particular 
colouration  depending,  in  the  case  of  the  di-  and  poly-hydric 
compounds,  on  the  relative  positions  of  the  hydroxyl-groups. 

o-Dihydroxy-compounds,  for  example,  give  with  ferric. 
chloride  a  green  colouration,  which  first  becomes  violet  - 
and  then  bright-red  on  addition  of  sodium  bicarbonate  ;  , 
TTHlihydroxy-compounds  give  a  deep  violet  colouration  ; 


390  PHENOLS. 

p-dihydroxy-com  pounds  give  a  green  colouration,  which  im- 
mediately changes  to  yellow  owing  to  the  formation  of  a 
quinone  (p.  413). 

All  phenols  give  Liebermann's  reaction ;  when  dissolved  in 
concentrated  sulphuric  acid  and  treated  with  a  nitroso-com- 
pound  or  a  nitrite,  they  yield  coloured  solutions,  which,  after 
diluting  and  adding  excess  of  alkali,  assume  an  intense  blue  or 
green  colour.  This  reaction,  therefore,  affords  a  convenient 
test  for  phenols  as  well  as  for  nitroso-compounds  (part  i. 
p.  204). 

Although  phenols  resemble  the  fatty  alcohols  and  the 
alcohols  of  the  aromatic  series  in  some  respects,  they  have, 
on  the  whole,  very  little  in  common  with  these  substances. 
The  reason  of  this  is,  that  the  character  of  the  hydroxyl-group 
(like  that  of  the  amido-group,  p.  359)  is  greatly  modified  by  its 
union  with  carbon  of  the  benzene  nucleus,  just  as  that  of  the 
hydroxyl-group  in  water  is  altered  by  combination  with  acid- 
forming  atoms  or  radicles  such  as  C1-,  N02-,  &c.,  as,  for 
example,  in  HOC1  and  H0-N02 ;  in  other  words,  the  phenolic 
hydroxyl-group  has  a  much  more  pronounced  acid  character 
than  that  in  alcohols,  a  fact  which  shows  that  the  radicles 
phenyl,  C6H5-,  phenylene,  C6H4<[,  &c.,  are  acid-forming 
radicles. 

The  acid  character  of  the  hydroxyl-group  in  phenols  is 
shown  in  their  behaviour  with  caustic  alkalies,  in  which  they 
dissolve  freely,  forming  metallic  derivatives  or  salts,  such  as 
sodium  plicnate,  C6H5-ONa,  potassium  cresate,  C6H4(CH3)-OK; 
these  compounds,  unlike  the  alkali  derivatives  of  the  alcohols, 
are  stable  in  presence  of  water,  but  are  decomposed  by  carbon 
dioxide  and  by  all  other  acids,  with  regeneration  of  the 
phenols.  For  this  reason  phenols  are  insoluble  in  alkali 
carbonates  unless  they  contain  other  acid-forming  groups  or 
atoms,  as,  for  example,  in  nitrophenol,  C6H4(N02)-OH,  and 
picric  acid,  C6H2(N02)3-OH,  when  their  acid  character  is 
often  enhanced  to  such  an  extent  that  they  decompose  alkali 
carbonates. 


PHENOLS.  391 

The  metallic  derivatives  of  the  phenols,  like  those  of  the 
alcohols,  interact  with  alkyl  halogen  compounds  and  with  acid 
chlorides,  yielding  substances  analogous  to  the  ethers  and 
ethereal  salts  respectively, 


C6H5-OK  +  CH3I  -  C6H6.0-CH3  +  KI 


oc  H 


NaBr 


2     5 

CHCOC1  =  C 


KC1; 

the  former,  like  the  ethers,  are  not  decomposed  by  boiling 
alkalies,  but  the  latter  readily  undergo  hydrolysis,  just  as  do 
the  ethereal  salts, 

KOH  =  C6H4<£Hs  +  C2H302K. 

•+ 

Towards  pentachloride  and  pentabromide  of  phosphorus, 
and  towards  acetic  anhydride  and  acetyl  chloride,  phenols 
behave  in  the  same  way  as  the  alcohols,  as  shown  by  the 
following  equations  : 

C6H5.OH  +  PC15  =  C6H5C1  +  POC13  +  HC1 
CeH5-OH  +  (CH3.CO)20  =  C6H5.O.CO-CH3  +  C2H4O2. 

Heating  with  halogen  'acids,  however,  does  not  change  the 
phenols  to  any  appreciable  extent,  because,  being  less  basic  in 
character  than  the  alcohols,  they  do  not  so  readily  form 
salts  with  mineral  acids. 

The  constitution  of  a  phenol  being  quite  different  from 
that  of  a  primary  or  secondary  alcohol,  the  fact  that  they  do 
not  yield  aldehydes  or  ketones  on  oxidation  was  only  to  be 
expected  ;  they  are,  however,  somewhat  similar  in  constitu- 
tion to  the  tertiary  alcohols,  and  like  the  latter,  they  often 
undergo  complex  changes  on  oxidation. 

Monohydric  Phenols. 

Phenol,  carbolic  acid,  or  hydroxybenzene,  C6H5-OH, 
occurs  in  very  small  quantities  in  human  urine  and  also  in 


392  PHENOLS. 

that  of  cows  ;  it  may  be  obtained  from  benzene,  nitrobenzene, 
aniline,  diazobenzene  chloride,  benzenesulphonic  acid,  and 
salicylic  acid  (p.  437)  by  the  methods  already  given,  but  the 
whole  of  the  phenol  of  commerce  is  prepared  from  coal-tar 
(compare  p.  297),  in  which  it  was  discovered  by  Runge 
in  1834. 

Phenol  crystallises  in  colourless,  deliquescent  prisms,  which 
melt  at  42°  and  turn  pink  on  exposure  to  air  and  light ;  it 
boils  at  183°,  and  is  volatile  in  steam.  It  has  a  very  character- 
istic smell,  is  highly  poisonous,  and  has  a  strong  caustic  action 
on  the  skin,  quickly  causing  blisters.  It  dissolves  freely  in 
most  organic  liquids,  but  is  only  sparingly  soluble  (1  part  in 
about  15)  in  cold  water;  its  aqueous  solution  gives  a  violet 
colouration  with  ferric  chloride,  and  a  pale-yellow  precipitate 
of  tribromophenol,  C6H2Br3-OH,  with  bromine  water;  both 
these  reactions  may  serve  for  the  detection  of  phenol.  Owing 
to  its  poisonous  and  antiseptic  properties,  phenol  is  extensively 
used  as  a  disinfectant ;  it  is  also  employed  in  large  quantities 
for  the  manufacture  of  picric  acid.  Potassium  phenate, 
C6H5-OK,  is  obtained  when  phenol  is  dissolved  in  potash  and 
the  solution  evaporated ;  it  is  a  crystalline  substance,  readily 
soluble  in  water,  and  is  decomposed  by  carbon  dioxide  with 
separation  of  phenol. 

Phenyl  methyl  ether,  or  anisole,  C6H5-0-CH3,  may  be  pre- 
pared by  heating  potassium  phenate  with  methyl  iodide ;  it  is 
a  colourless  liquid,  boiling  at  155°,  and  is  similar  to  the  ethers 
of  the  fatty  series  in  chemical  properties,  although  it  also 
shows  the  usual  behaviour  of  aromatic  compounds,  and  readily 
yields  nitro-derivatives,  &c.  When  warmed  with  concen- 
trated hydriodic  acid,  it  yields  phenol  and  methyl  iodide, 

C6H5:0-CH3  +  HI  =  C6H5-OH  +  CH3L 

Phenyl  ethyl  ether,  or  phenetole,  C6H5-0-C2H5,  can  be 
obtained  in  a  similar  manner  ;  it  boils  at  172°. 

Nitrophenols,  C6H4(M)2)-OH,  are  formed  very  readily  on 
treating  phenol  even  with  dilute  nitric  acid,  the  presence  of 


PHENOLS  393 

the  hydroxyl-groiip  not  only  facilitating  the  introduction  of 
the  nitro-group,  but  also  determining  the  position  taken  up  by 
the  latter.  When  phenol  is  gradually  added  to  nitric  acid  of 
sp.  gr.  1-11  (6  parts),  the  mixture  being  kept  cold  and  fre- 
quently shaken,  it  is  converted  into  a  mixture  of  o-  and  p-nitro- 
phenol,  which  separates  as  a  dark-brown  oil  or  resinous  mass ; 
this  product  is  allowed  to  settle,  washed  with  water  by  de- 
cantation,  and  then  submitted  to  distillation  in  steam,  where- 
upon the  ortlw-nitrophenol  passes  over  as  a  yellow  oil,  which 
crystallises  on  cooling ;  the  oily  residue  in  the  flask  is  mixed 
with  a  little  more  water,  the  mixture  heated  to  boiling,  and 
the  hot  solution  filtered  from  tarry  matter,  the  para-nitro- 
phenol  which  separates  on  cooling  being  purified  by  recrystal- 
lisation  from  boiling  water  with  addition  of  animal  charcoal.* 
Meta-nitrophenol  is  prepared  by  reducing  meta-dinitrobenzene 
to  meta-nitraniline  (p.  363),  and  treating  a  solution  of  the  latter 
in  excess  of  dilute  sulphuric  acid  with  nitrous  acid ;  the 
solution  of  the  diazo-salt  is  then  slowly  heated  to  boiling,  and 
the  meta-nitrophenol  thus  produced  purified  by  recrystallisa- 
tion  from  water. 

The  melting-points  of  the  three  compounds  are  : 

Ortho-nitrophenol,  Meta-nitrophenol,  Para-nitrophenol, 

45°.  96°.  114°. 

The  o-  and  the  ??i-compounds  are  yellow,  but  the  ^-derivative 
is  colourless  ;  only  the  o-compound  is  volatile  in  steam.  The 
three  compounds  are  all  sparingly  soluble  in  cold  water,  but 
dissolve  freely  in  alkalies  and  also  in  alkali  carbonates,  forming 

*  Animal  charcoal  is  prepared  by  strongly  heating  blood  or  bones  out  of 
contact  with  air ;  it  is  frequently  used  in  the  purification  of  organic  com- 
pounds, as  it  has  the  property  of  absorbing  coloured  impurities  from  solu- 
tions. For  this  purpose  the  dark-coloured,  impure  substance  is  dissolved 
in  water,  ether,  alcohol,  benzene,  or  some  other  solvent,  a  small  quantity  of 
animal  charcoal  added,  and  the  mixture  heated  for  some  time  with  reflux 
condenser  (part  i.  p.  186)  ;  on  subsequently  filtering,  a  colourless  or  a  much 
lighter  coloured  solution  is  usually  obtained.  Before  use,  the  charcoal 
should  be  repeatedly  extracted  with  boiling  hydrochloric  acid,  washed  well, 
dried,  and  heated  strongly  in  a  porcelain  crucible  closed  with  a  lid. 


394  PHENOLS. 

dark-yellow  or  red  salts  which  are  not  decomposed  by 
carbon  dioxide ;  they  have,  therefore,  a  more  marked  acid 
character  than  phenol  itself,  the  presence  of  the  nitro-group 
having  an  effect  comparable  to  that  of  the  nitro-group  in  nitric 
acid,  H0-N02. 

Picric  acid,  or  trinitrophenol,  C6H2(N02)3-OH,  is  formed 
when  substances  such  as  wool,  silk,  leather,  and  resins  are 
heated  with  concentrated  nitric  acid,  very  complex  reactions 
taking  place  ;  it  may  be  obtained  by  heating  phenol,  or  the  o- 
and  ^9-nitrophenols,  with  nitric  acid,  but  the  product  is  not 
very  easily  purified  from  resinous  substances  which  are  formed 
at  the  same  time.  For  this  reason  picric  acid  is  best  prepared 
by  dissolving  phenol  (1  part)  in  an  equal  weight  of  concen- 
trated sulphuric  acid,  and  adding  this  solution  to  nitric  acid  of 
sp.  gr.  1-4  (3  parts)  in  small  quantities  at  a  time  ;  after  the 
first  energetic  action  has  subsided,  the  mixture  is  carefully 
heated  on  a  water-bath  for  about  two  hours.  On  cooling,  the 
product  solidifies  to  a  mass  of  crystals,  which  are  collected, 
washed,  and  recrystallised  from  hot  water. 

When  phenol  is  dissolved  in  sulphuric  acid,  it  is  converted  into  a 
mixture  of  o-  and  p-phenolsulphonic  acids,  C6H4(OH)-SO3H  (see 
below);  on  subsequent  treatment  with  nitric  acid,  the  sulphonic 
group,  as  well  as  two  atoms  of  hydrogen,  are  displaced  by  nitro- 
groups, 

C6H4<°^H  +  3HO-N02=  C6H2(N02)3.OH  +  H2S04  +  2H20. 

Picric  acid  is  a  yellow  crystalline  compound,  melting  at 
122-5°.  It  is  only  very  sparingly  soluble  in  cold,  but 
moderately  easily  in  hot,  water,  and  its  solutions  dye  silk  and 
wool  (not  cotton,  p.  502)  a  beautiful  yellow  colour ;  it  is,  in  fact, 
one  of  the  earliest  known  artificial  organic  dyes.  It  has  very 
marked  acid  properties,  and  readily  decomposes  carbonates. 
The  potassium  derivative,  C6H2(N02)3-OK,  and  the  sodium 
derivative,  C6H2(N02)3-ONa,  are  yellow  crystalline  com- 
pounds, the  former  being  sparingly,  the  latter  readily  soluble 
in  cold  water.  These  compounds,  and  also  the  ammonium 


PHENOLS.  395 

derivative,  explode  violently  on  percussion  or  when  heated, 
and  are  employed  in  the  preparation  of  explosives ;  picric 
acid  itself  burns  quietly  when  ignited,  but  can  be  caused  to 
explode  violently  with  a  detonator. 

Picric  acid  may  be  produced  by  oxidising  symmetrical  trinitro- 
benzene,  C6H3(NO2)3,  with  potassium  ferricyanide,  the  presence  of 
the  nitre-groups  facilitating  the  substitution  of  hydroxyl  for 
hydrogen ;  as,  moreover,  it  is  quite  immaterial  which  of  the  three 
hydrogen  atoms  is  displaced,  since  they  all  occupy  a  similar  position 
relatively  to  the  rest  of  the  molecule,  the  constitution  of  picric  acid 
must  be  represented  by  the  formula 


OH 


or,  for  the  sake  of  convenience,  the  relative  positions  of  the  several 

1  2  4  (3 

groups  may  be  indicated  in  this  way  [OH  :  NO2 :  NO2 :  NO2] ;  it 
would,  of  course,  be  just  the  same  if  the  groups  were  numbered 

[NO2 :  OH  :  N02  :  N02]  or  [N02  :  N02 :  OH  :  N02],  since  the  relative 
positions  are  the  same  in  the  three  cases,  and  it  is  of  no  consequence 
at  which  carbon  atom  the  numbering  commences. 

Picric  acid  has  the  curious  property  of  forming  crystalline  com- 
pounds with  benzene,  naphthalene,  anthracene,  and  many  other 
hydrocarbons,  so  that  it  is  sometimes  used  in  detecting  and  also  in 
purifying  small  quantities  of  the  substances  in  question  ;  the  com- 
pound which  it  forms  with  benzene,  for  example,  crystallises  in 
yellow  needles,  is  decomposed  by  water,  and  has  the  composition 
C6H2(N02)3-OH,  C6H6. 

Phenol-o-sulpJiomc  acid,  C6H4(OH)-S03H,  is  formed,  to- 
gether with  a  comparatively  small  quantity  of  the  p-acid, 
when  a  solution  of  phenol  in  concentrated  sulphuric  acid  is 
kept  for  some  time  at  ordinary  temperatures ;  if,  however, 
the  solution  be  heated  at  100-110°,  the  o-acid,  which  is  the 
primary  product,  is  gradually  converted  into  phenol-p-sulphonic 
acid. 

Plienol-m-sulplwnic   acid  is  prepared  by  carefully  heating 


396  PHENOLS. 

benzene-w-disulphonic  acid  with  potash  at  170-180°;  under 
these  conditions  only  one  of  the  sulphonic  groups  is  displaced, 

2KOH  =  CA<S03K 

The  o-acid  is  interesting  on  account  of  the  fact  that  it  is 
converted  into  the  £>-acid  when  boiled  with  water,  and  also 
because  it  is  used  as  an  antiseptic  under  the  name  aseptol. 

The  three  (o.m.p.)ciesols  or  hydroxy  toluenes,  C6H4(CH3)  -OH, 
the  next  homologues  of  phenol,  occur  in  coal-tar,  but  cannot 
be  conveniently  isolated  from  this  source  owing  to  the 
difficulty  of  separating  them  from  one  another  ;  they  are 
prepared  from  the  corresponding  toluidines  or  arnidotoluenes, 
C6H4(CH3)-NH2,  by  means  of  the  diazo-reaction,  or  by  fusing 
the  corresponding  toluenesulphonic  acids  with  potash, 

C6H4<so  K  +  KOH  =  CA<o§3  +  KsS03. 

3 

They  resemble  phenol  in  most  ordinary  properties,  as,  for 
example,  in  being  sparingly  soluble  in  water,  and  in  forming 
potassium  and  sodium  derivatives,  which  are  decomposed 
by  carbon  dioxide  ;  they  also  yield  alkyl-derivatives,  &c.,  by 
the  displacement  of  the  hydrogen  of  the  hydroxyl-group. 
On  distillation  with  zinc-dust  they  are  all  converted  into 
toluene, 

+  Zn  =  C6H5.CH,  +  ZnO, 


and  they  all  give  a  bluish  colouration  with  ferric  chloride. 

One  very  curious  fact  regarding  the  three  cresols  is  that 
they  are  not  oxidised  by  chromic  acid,  although  toluene,  as 
already  stated,  is  slowly  converted  into  benzoic  acid  ;  the 
presence  of  the  hydroxyl-group,  therefore,  protects  the  methyl- 
group  from  the  attack  of  acid  oxidising  agents,  and  this  is  true 
also  in  the  case  of  other  phenols  of  similar  constitution.  If, 
however,  the  hydrogen  of  the  hydroxyl-group  be  displaced 
by  an  alkyl,  or  by  an  acid  group  such  as  acetyl,  then 
the  protection  is  withdrawn,  and  the  methyl-group  is 


PHENOLS.  397 

converted  into  the  carboxyl-group  in  the  usual  manner  • 
the  methylcresols,  C6H4(OCH3)-CH3,  for  example,  are  oxidised 
by  chromic  acid,  yielding  the  corresponding  methoxybenzoic 
acids,  C6H4(OCH3).COOH. 

The  melting  and  boiling  points  of  the  three  cresols  are 
given  below : 

Ortho-cresol.  Meta-cresol.  Para-cresol. 

M.p.  31 J  5°  36° 

B.p.  188°  201°  198° 

Of  the  higher  monohydric  phenols,  thymol  and  carvacrol 
may  be  mentioned ;  these  two  compounds  are  isomeric  mono- 
hydroxy-derivatives  of  cymene,  C6H4(CH3)-C3H7  (p.  339),  and 

their  constitutions  are  respectively  represented  by  the  formulae 

CH3 


—OH 


—OH 


CH(CH3>2  CH(CH3)2. 

Thymol.  Carvacrol. 

Thymol  occurs  in  oil  of  thyme,  together  with  cymene ;  it 
crystallises  in  large  plates,  melts  at  51-5°,  and  has  a  charac- 
teristic smell  like  that  of  thyme.  It  is  only  very  sparingly 
soluble  in  water,  and  does  not  give  a  colouration  with  ferric 
chloride ;  when  heated  with  phosphoric  anhydride,  it  yields 
propylene  and  ?n-cresol, 

C6H3(OH)<^  =  C6H4(OH).CH3  +  CSH6. 

Carvacrol   occurs  in  the    oil  of   Origanum  hirtum,  and  is 
easily  prepared  by  heating  camphor  with  iodine, 
C10H160  +  I2  -  C10H140  +  2HI ; 

it  is  an  oil  boiling  at  237°,  and*its  alcoholic  solution  gives  a 
green  colouration  with  ferric  chloride.  When  heated  with 
phosphoric  anhydride,  it  is  decomposed  into  propylene  and 
o-cresol. 


398  PHENOLS. 

Dihydric  Phenols. 

The  isomeric  dihydric  phenols — catechol,  resorcinol,  and 
hydroquinone — are  well-known  compounds  of  considerable 
importance,  and  are  respectively  represented  by  the  formulae 


'"    i    i 

-OH 


)H 

Catechol,  Resorcinol,  Hydroquinone, 

or  or  or 

Ortho-dihydroxybenzene.     Meta-dihydroxybenzene.     Para-dihydroxybenzene. 

Catechol,  or  pyrocatechin,  C6H4(OH)2,  occurs  in  catechu, 
a  substance  obtained  in  India  from  Acacia  catechu  and  other 
trees,  and  was  first  obtained  by  the  dry  distillation  of  this 
vegetable  product ;  it  may  be  obtained  by  fusing  phenol -o- 
sulphonic  acid,  C6H4(OH)-S03H,  with  potash,  but  is  most 
conveniently  prepared  by  heating  guaiacol  or  methylcatechol 
(a  colourless  liquid,  boiling  at  200°,  obtained  from  the  tar  of 
beechwood),  with  concentrated  hydriodic  acid, 

It  is  a  colourless,  crystalline  substance,  melting  at  104°,  and 
is  readily  soluble  in  water ;  its  aqueous  solution  gives,  with 
ferric  chloride,  a  green  colouration,  which,  on  the  addition  of 
sodium  bicarbonate,  changes  first  to  violet  and  then  to  red,  a 
reaction  which  is  common  to  all  or^o-dihydric  phenols  (p.  389). 
Guaiacol  shows  a  similar  behaviour  with  ferric  chloride,  but 
when  the  hydrogen  atoms  of  both  the  hydroxyl-groups  are 
displaced,  as,  for  example,  in  dimethylcatechol  or  veratrol, 
C6H4(OCH3)2,  there  is  no  colouration. 

Resorcinol,  C6H4(OH)2,  is  prepared  on  a  large  scale  by 
fusing  benzene-m-disulphonic  acid  with  potash, 

•V  +  2KOH  -  C61 
8*- 


PHENOLS.  399 

but  it  is  also  obtained  when  the  para-disulphonic  acid,  and 
many  other  ortho-  and  para-derivatives  of  benzene  are  treated 
in  the  same  way,  owing  to  intramolecular  change  taking  place 
(compare  p.  388).  It  is  a  crystalline  substance,  melting  at 
110°,  and  dissolves  freely  in  water,  alcohol,  and  ether;  its 
aqueous  solution  gives  a  dark-violet  colouration,  with  ferric 
chloride  and  a  crystalline  precipitate  of  tribromoresorcinol, 
C6HBr3(OH)9,  with  bromine  water.  When  resorcinol  is 
strongly  heated  for  a  few  minutes  with  phthalic  anhydride 
(p.  426),  or  with  the  anhydride  of  some  other  dicarboxylic 
acid  (succinic  anhydride,  for  example),  and  the  yellowish-red 
mass  then  dissolved  in  dilute  soda,  a  yellowish-brown  solution, 
which  shows  a  beautiful  green  fluorescence,  is  obtained ;  this 
phenomenon  is  due  to  the  formation  of  &  fluorescein  (p.  520). 
Other  ?ft-dihydric  phenols  give  this  fluorescein  reaction,  which, 
therefore,  affords  a  convenient  and  very  delicate  test  for  such 
compounds  ;  the  fluorescein  reaction  may  also  be  employed  as 
a  test  for  anhydrides  of  dicarboxylic  acids. 

Resorcinol  is  used  in  large  quantities  in  preparing  fluorescein, 
eosin,  and  azo-dyes. 

Hydroquinone,  or  quinol,  C6H4(OH)2,  is  formed,  together 
with  glucose,  when  the  glucoside,  arbutin — a  substance  which 
occurs  in  the  leaves  of  the  bear-berry — is  boiled  with  water, 

C12H1607  +  H20  =  C6H4(OH)2  +  C6H1206. 

It  is  usually  prepared  by  reducing  quinone  (p.  413)  with 
sulphurous  acid  in  aqueous  solution,  and  then  extracting  with 
ether, 

C6H402  +  H2SOS  +  H20  =  C6H4(OH)2  +  H2S04. 
It  melts  at  169°,  is  readily  soluble  in  water,  and  when  treated 
with  ferric  chloride  or  other  mild  oxidising  agents,  it  is  con- 
verted into  quinone, 

C6H4(OH)2  +  0  =  C6H402  +  H20. 

Trihydric  Phenols. 
The   three    trihydric    phenols,    C6H3(OH)3,    which    should 


400  PHENOLS. 

exist  in  accordance  with  theory,  are  all  known,  and  are  re- 
spectively represented  by  the  following  formulae  : 


OH 


—OH 
—OH 


Pyrogallol,  Phloroglucinol,  Hydroxyhydroquinone 

1:2:  3-Trihydroxybenzene.      1:3:  5-Trihydroxy  benzene.      1:2:  4-Trihydroxybenzene. 

Pyrogallol,  C6H3(OH)3,  sometimes  called  pyrogallic  acid,  is 
prepared  by  heating  gallic  acid  (p.  439)  alone  or  with  glycerol, 
at  about  210°,  until  the  evolution  of  carbon  dioxide  ceases, 

C6H2(OH)3.COOH  =  C6H3(OH)3  +  C02. 

It  is  a  colourless,  crystalline  substance,  melting  at  115°, 
and  is  readily  soluble  in  water,  but  more  sparingly  in 
alcohol  and  ether  (the  effect  of  hydroxyl-groups)  ;  its  aqueous  - 
solution  gives,  with  ferric  chloride,  a  red,  and  with  ferrous 
sulphate  containing  a  trace  of  ferric  chloride,  a  deep,  dark- 
blue  colouration.  It  dissolves  freely  in  alkalies,  giving 
solutions  which  rapidly  absorb  oxygen  and  turn  black  on 
exposure  to  the  air,  a  fact  which  is  made  use  of  in  gas 
analysis  for  the  estimation  of  oxygen.  Pyrogallol  has  power- 
ful reducing  properties,  and  precipitates  gold,  silver,  and 
mercury  from  solutions  of  their  salts,  being  itself  oxidised 
to  oxalic  and  acetic  acids  ;  many  other  phenols,  such  as 
catechol,  resorcinol,  and  hydroquinone,  show  a  similar 
behaviour,  especially  in  alkaline  solution,  but  the  monohydric- 
compounds  are  much  less  readily  oxidised,  and  consequently 
do  not  exhibit  reducing  properties.  Pyrogallol  and  hydro- 
quinone are  used  in  photography  as  developers. 

Like  glycerol  and  other  trihydric-compounds,  pyrogallol 
forms  mono-,  di-,  and  tri-alkyl-derivatives,  such  as 
C6H3(OH)2.OC2H5,  C6H3(OH)(OC2H5)2,  and  C6H3(OC2H5)3  ; 
the  dimethyl-derivative,  C6H3(OCH3)2-OH,  occurs  in  beech- 
wood  tar. 


PHENOLS.  401 

Phloroglucinol,  or  symmetrical  trihydroxy  benzene, 
C6H3(OH)3,  is  produced  when  phenol,  resorcinol,  and  many 
resinous  substances,  such  as  gamboge,  dragon's-blood,  &c.,  are 
fused  with  potash. 

It  is  best  prepared  by  fusing  resorcinol  (1  part)  with  soda  (6 
parts)  for  about  twenty-five  minutes,  or  until  the  vigorous  evolu- 
tion of  hydrogen  has  ceased  ;  the  chocolate-coloured  melt  is  dis- 
solved in  water,  acidified  with  sulphuric  acid,  extracted  with 
ether,  the  ethereal  extract  evaporated,  and  the  residue  recrystal- 
lised  from  water. 

It  crystallises  in  colourless  prisms,  melts  at  about  218°,  and 
is  very  soluble  in  water ;  the  solution,  which  has  a  sweet 
taste,  gives,  with  ferric  chloride,  a  bluish-violet  colouration, 
and  when  mixed  with  potash,  it  rapidly  turns  brown  in  con- 
tact with  air  owing  to  absorption  of  oxygen.  When  digested 
with  acetyl  chloride,  phloroglucinol  yields  a  triacetate, 
C6H3(C2H309)3  melting  at  106°,  and  in  many  other  reactions 
it  shows  properties  in  harmony  with  the  formula 


On  the  other  hand,  when  treated  with  hydroxylamine,  it  gives 
a  trioxime,  C6H6(N-OH)3,  and  in  this  and  other  respects  it 
behaves  as  though  it  were  a  triketone  of  the  constitution 

o 


H2 

Possibly,  therefore,  phloroglucinol  is  capable  of  existing  in 
two  forms,  which  are  convertible,  the  one  into  the  other,  by 
intramolecular  change  (part  i.  p.  195). 

Hydroxykydroquinone,  or  trihydroxybenzene,  (1:2:4),  is  formed 
when  hydroquinone  is  fused  with  potash.  It  melts  at  140°,  and  is 
very  soluble  in  water,  but  its  aqueous  solution  is  coloured  greenish- 
brown  by  ferric  chloride,  but  on  the  addition  of  sodium  carbonate  the 
colour  changes  to  blue  and  then  to  red  (p.  389). 

Z 


AROMATIC   ALCOHOLS,    ETC. 


CHAPTER    XXVII. 


AROMATIC   ALCOHOLS,  ALDEHYDES,   KETONES,  AND  QUINONES. 

Alcohols. 

The  aromatic  alcohols  are  derived  from  the  hydrocarbons  by 
substituting  hydroxy-groups  for  hydrogen  atoms  of  the  side- 
chain  ;  benzyl  alcohol,  C6H5-CH2-OH,  for  example,  is  derived 
from  toluene,  tolyl  alcohol,  C6H4(CH3).CH2-OH,  from  xylene, 
and  so  on.  The  compounds  of  this  kind  have  not  been  very 
fully  investigated,  but  from  what  is  known  of  their  properties, 
it  is  clear  that  they  are  very  closely  related  to  the  alcohols  of 
the  fatty  series,  although,  of  course,  they  show  at  the  same 
time  the  general  behaviour  of  aromatic  substances. 

They  may  be  prepared  by  methods  exactly  analogous  to 
those  employed  in  the  case  of  the  fatty  alcohols — namely,  by 
heating  the  corresponding  halogen  derivatives  with  water, 
weak  alkalies,  or  silver  hydroxide, 

C6H5.CH2C1  +  H20  =  C6H5.CH2.OH  +  HC1, 
and  by  reducing  the  corresponding  aldehydes  and  ketones, 
C6H5.CH2.CHO  +  2H  =  C0H5.CH2.CH0.OH 
C6H5.CO-CH3  +  2H  =  C6H6.CH(OH).CH3. 

Those  compounds  which,  like  benzyl  alcohol,  contain  the 
carbinol  group,  -CH2-OH,  directly  united  with  the  benzene 
nucleus,  may  also  be  prepared  by  treating  the  corresponding 
aldehydes  with  potash  (compare  p.  408), 

2C6H5.CHO  +  H20  -  C6H5.CH2.OH  +  C6H5-COOH. 

The  aromatic  alcohols  are  usually  colourless  liquids  or 
solids,  sparingly  soluble  in  water ;  their  behaviour  with  alkali 
metals,  phosphorus  pentachloride,  and  acids,  is  similar  to  that 
of  the  fatty  compounds,  as  will  be  seen  from  a  consideration 
of  the  properties  of  benzyl  alcohol,  one  of  the  few  well-known 
aromatic  alcohols. 


AROMATIC    ALCOHOLS,    ETC.  403 

Benzyl  alcohol,  phenylcarbinol,  or  hydroxytoluene, 
C6H5.CH2-OH,  an  isomeride  of  the  three  cresols  (p.  396), 
occurs  in  storax  (a  resin  obtained  from  the  tree  Styrax 
officinalis),  and  also  in  balsam  of  Peru  and  balsam  of  Tolu, 
either  in  the  free  state  or  as  ethereal  salts  in  combination 
with  cinnamic  and  benzoic  acids. 

It  may  be  obtained  by  reducing  benzaldehyde  (p.  405)  with 
sodium  amalgam, 

C6H5.CHO  +  2H  =  C6H5.CH2.OH, 

and  by  boiling  benzyl  chloride  with  a    solution   of   sodium 
carbonate, 

C6H5.CH2C1  +  H20  =  C6H5.CH2.OH  +  HC1; 

but  it  is  most  conveniently  prepared  by  treating  benzaldehyde 
with  cold  potash, 

2C6H5.CHO  +  H20  =  C6H5.CH2-OH  +  C6H5-COOH. 

The  aldehyde  (10  parts)  is  shaken  with  a  solution  of  potash  (9 
parts)  in  water  (10  parts)  until  the  whole  forms  an  emulsion,  which 
is  then  allowed  to  stand  for  twenty-four  hours  ;  after  adding  water 
to  dissolve  the  potassium  benzoate,  the  solution  is  extracted  with 
ether,  the  ethereal  extract  evaporated,  and  the  benzyl  alcohol 
purified  by  distillation. 

Benzyl  alcohol  is  a  colourless  liquid,  boiling  at  206° ;  it  is 
only  sparingly  soluble  in  water,  but  miscible  with  alcohol, 
ether,  &c.,  in  all  proportions.  It  dissolves  sodium  and 
potassium  with  evolution  of  hydrogen,  yielding  metallic 
derivatives  which  are  decomposed  by  water,  and,  when 
treated  with  phosphorus  pentachloride,  it  is  converted  into 
benzyl  chloride, 

C6H5-CH2.OH  +  PC15  =  C6H5.CH2C1  +  POC13  +  HCL 

When  heated  with  concentrated  acids,  or  treated  with 
anhydrides  or  acid  chlorides,  it  gives  ethereal  salts;  with 
hydrobromic  acid,  for  example,  it  yields  benzyl  bromide, 
C6H5-CH2Br  (b.p.  199°),  and  with  acetyl  chloride  or  acetic 
anhydride  it  gives  benzyl  acetate,  C6H5.CH2-()-CO-CH3  (b.p. 


404  AROMATIC   ALCOHOLS,    ETC. 

206°).      On   oxidation  with   dilute   nitric   acid,    it    is    first 
converted  into  benzaldehyde  and  then  into  benzoic  acid, 

C6H5-CH2.OH  +  0  =  C6H5.CHO  +  H20 
C6H5.CH2-OH  +  20  =  C6H5-COOH  +  H20. 

All  these  changes  are  strictly  analogous  to  those  undergone 
by  the  fatty  alcohols. 

Saligenin,  C6H4(OH)-CH2>OH,  also  known  as  o-hydroxybenzyl 
alcohol,  or  salicyl  alcohol,  is  an  example  of  a  substance  which  is 
both  a  phenol  and  an  alcohol.  It  is  produced  by  the  action  of 
dilute  acids  or  ferments  on  salicin  (a  glucoside  existing  in  the  bark 
of  the  willow-tree),  which  breaks  up  into  saligenin  and  dextrose, 

Ci3H1807  +  H2O  =  C6H4<C£.jj  .QJJ  +  C6H1206. 

Synthetically,  it  may  be  prepared  by  reducing  salicylaldehyde 
(p.  409)  with  sodium  amalgam, 

C6H4<CHO  +  2H  =  C6H4<CH2  OH. 

Saligenin  is  a  crystalline  substance  which  melts  at  82°,  and  is 
readily  soluble  in  water,  the  solution  acquiring  a  deep  blue 
colouration  on  the  addition  of  ferric  chloride.  Owing  to  its 
phenolic  nature,  it  forms  alkali  salts,  which,  when  heated  with 
alkyl  halogen  compounds,  give  the  corresponding  ethers  (the  methyl 
ether,  C6H4(OCH3)-CH2-OH,  is  a  colourless  oil,  boiling  at  247°)  ;  on 
the  other  hand,  it  shows  the  properties  of  an  alcohol,  and  yields 
salicylaldehyde  and  salicylic  acid  on  oxidation. 

The  m-  and  p-hydroxybenzyl  alcohols  may  be  prepared  by  the 
reduction  of  the  m-  and  jo-hydroxybenzaldehydes  (p.  410)  ;  they  are 
colourless,  crystalline  substances,  which  melt  at  67°  and  110°  re- 
spectively. 

Anisyl  alcohol,  or  p-methoxybenzyl  alcohol,  C6H4(OCH3)-CH2-OH, 
is  obtained  by  treating  anisaldehyde,  C6H4(OCH3)-CHO  (p.  410), 
with  sodium  amalgam  or  with  alcoholic  potash.  Synthetically,  it 
has  been  prepared  by  heating  a  mixture  of  p-hydroxybenzyl 
alcohol,  potash,  and  methyl  iodide  in  alcoholic  solution  at  100°, 

.        +  CH3I  =  C6H4<  +  KI. 


It  is  a  crystalline  solid,  which  melts  at  25°  and  boils  at  258°  ;   on 
oxidation,  it  yields  anisaldehyde  and  anisic  acid,  C6H4(OCH3)-COOH. 


AROMATIC    ALCOHOLS,    ETC.  405 

Aldehydes. 

The  relation  between  the  aromatic  aldehydes  and  the 
aromatic  alcohols  is  the  same  as  that  which  exists  between 
the  corresponding  classes  of  fatty  compounds — that  is  to  say, 
the  aldehydes  are  derived  from  the  primary  alcohols  by 
taking  away  two  atoms  of  hydrogen  from  the  -CH2-OH  group; 
benzaldehyde,  C6H5-CHO,  for  example,  corresponds  with  benzyl 
alcohol,  C6H5  •  CH2  •  OH,  salicylaldehyde,  C6H4(OH)  -  CHO, 
with  salicyl  alcohol,  C6H4(OH).CH2-OH,  phenylacetaldehyde, 
C6H5.CH2.CHO,  with  phenylethyl  alcohol,  C6H5.CH2-CH2.OH, 
and  so  on. 

Now  those  compounds  which  contain  an  aldehyde-group 
directly  united  with  carbon  of  the  nucleus  have  been  much 
more  thoroughly  investigated,  and  are  of  far  greater  import- 
ance, than  those  in  which  the  aldehyde-group  is  combined  with 
a  carbon  atom  of  the  side-chain,  as  in  pheriylacetaldehyde 
(see  above),  cinnamic  aldehyde,  C6H5-CH:CH-CHO,  &c. ; 
whereas,  moreover,  the  latter  resemble  the  fatty  aldehydes 
very  closely  in  general  character,  and  do  not  therefore  require 
any  detailed  description,  the  former  differ  from  the  fatty  com- 
pounds in  several  important  particulars,  as  will  be  seen  from 
the  following  account  of  benzaldehyde  and  salicylaldehyde, 
two  of  the  best-known  aromatic  compounds  which  contain 
the  aldehyde  group  directly  united  with  the  benzene  nucleus. 

Benzaldehyde,  C6H5-CHO,  sometimes  called  'oil  of  bitter 
almonds,'  was  formerly  obtained  from  the  glucoside  (compare 
foot-note,  p.  488),  amygdalin,  which  occurs  in  bitter  almonds, 
and  which,  in  contact  with  water,  gradually  undergoes  de- 
composition into  benzaldehyde,  hydrocyanic  acid,  and  dextrose 
(compare  part  i.  p.  279). 

Benzaldehyde  may  be  obtained  by  oxidising  benzyl 
alcohol  with  nitric  acid,  and  by  distilling  a  mixture  of  calcium 
benzoate  and  calcium  formate, 

(C6H5-COO)2Ca  +  (H.COO)2Ca  ==  2C6H5-CHO  +  2CaC03, 
reactions  analogous  to  those  employed  in  the  fatty  series. 


406  AROMATIC   ALCOHOLS,    ETC. 

It  is  prepared  both  in  the  laboratory  and  on  the  large  scale, 
either  by  heating  benzal  chloride  (p.  349)  with  moderately 
dilute  sulphuric  acid,  or  calcium  hydroxide,  under  pressure, 
or  by  boiling  benzyl  chloride  with  an  aqueous  solution  of  lead 
nitrate  or  copper  nitrate.  In  the  first  method,  the  benzal 
chloride  is  probably  first  converted  into  the  corresponding 
dihydroxy-derivative  of  toluene, 

C6H5.CHCJ2  +  2H20  =  C6H5-CH(OH)2  +  2HC1; 

but  as  this  compound  contains  two  hydroxyl-groups  united 
with  one  and  the  same  carbon  atom,  it  is  very  unstable  (part  i. 
p.  259),  and  subsequently  undergoes  decomposition  into 
benzaldehyde  and  water.  In  the  second  method,  the  benzyl 
chloride  is  probably  transformed  into  benzyl  alcohol,  which 
is  then  oxidised  to  the  aldehyde  by  the  metallic  nitrate,  with 
evolution  of  oxides  of  nitrogen  and  formation  of  copper  or 
lead  chloride,  as  indicated  by  the  equation 

2C6H5.CH2-OH  +  Cu(NOs)9  +  2HC1  = 


2C6H6.CHO  +  CuCljj  +  2HN02  +  2H20. 

Benzyl  chloride  (5  parts),  water  (25  parts),  and  copper  nitrate 
(4  parts)  are  placed  in  a  flask  connected  with  a  reflux  condenser, 
and  the  mixture  is  boiled  for  six  to  eight  hours,  a  stream  of  carbon 
dioxide  being  passed  into  the  liquid  all  the  time,  in  order  to  expel 
the  oxides  of  nitrogen,  which  would  otherwise  oxidise  the  benzal- 
dghyde  to  benzole  acid  ;  the  process  is  at  an  end  when  the  oil 
contains  only  traces  of  chlorine,  which  is  ascertained  by  washing 
a  small  portion  with  water,  and  boiling  it  with  silver  nitrate 
and  nitric  acid.  The  benzaldehyde  is  then  extracted  with  ether, 
the  ethereal  extract  shaken  with  a  concentrated  solution  of 
sodium  bisulphite,  and  the  crystals  of  the  bisulphite  compound, 
C6H5-CHO,  NaHSO3,  separated  by  filtration  and  washed  with  ether  ; 
the  benzaldehyde  is  then  regenerated  by  decomposing  the  crystals 
with  dilute  sulphuric  acid,  extracted  with  ether,  and  distilled. 

Benzaldehyde  is  a  colourless,  highly  refractive  liquid  of  sp.  gr. 
1-05  at  15°  ;  it  boils  at  179°,  and  is  volatile  in  steam.  It  has 
a  pleasant  smell  like  that  of  bitter  almonds,  and  is  only 
sparingly  soluble  in  water,  but  miscible  with  alcohol,  ether, 
&c.,  in  all  proportions.  It  is  extensively  used  for  flavouring 


AROMATIC   ALCOHOLS,    ETC.  407 


purposes,  and  is  employed  on  the  large  scale  in  the  manu- 
facture of  various  dyes. 

Benzaldehyde,  and  aromatic  aldehydes  in  general,  resemble 
the  fatty  aldehydes  in  the  following  respects :  They  readily 
undergo  oxidation  on  exposure  to  the  air,  yielding  the  corre- 
sponding acids, 

C6H5-CHO  +  0  -  C6H5-COOH, 

and  consequently  they  reduce  ammoniacal  solutions  of  silver 
hydroxide.  On  reduction,  they  are  converted  into  the 
corresponding  alcohols, 

C6H5-CHO  +  2H  =  C6H5.CH2.OH. 

When  treated  with  phosphorus  pentachloride,  they  give 
dihalogen  derivatives  such  as  benzal  chloride,  CgHg-CHCl^ 
two  atoms  of  chlorine  being  substituted  for  one  atom  of 
oxygen.  They  interact  with  hydroxylamine,  yielding 
aldoximes,  and  with  phenylhydrazine,  giving  hydrazones, 

C6H5-CHO  +  NH2.OH  =  H20  +  C6H5.CH:N.OH 

Benzaldoxime. 

C6H5-CHO  +  NH2.NH-C6H5  -  H20  +  C6H5.CH:]Sr2H.C6H5. 

Benzylidenehydrazone. 

They  combine  directly  with  sodium  bisulphite,  forming 
crystalline  compounds,  and  with  hydrocyanic  acid  they 
yield  hydroxy cyanides  such  as  benzylidenehydroxycyanide,* 

C6H5-CH<^p^-     They  readily   undergo  condensation  with 

many  other  fatty  and  aromatic  compounds;  when,  for 
example,  a  mixture  of  benzaldehyde  and  acetone  is  treated 
with  a  few  drops  of  soda  at  ordinary  temperatures,  condensa- 
tion occurs,  and  benzylideneacetone,  C6H5*CH:CILCO-CH3 
(m.p.  42°),  is  formed. 

Benzaldehyde,  and  other  aromatic  aldehydes  which  contain 
the  -CHO  group  directly  united  with  the  benzene  nucleus, 
differ  from  the  fatty  aldehydes  in  the  following  respects : 

*  The  name  henzylidene  is  given  to  the  group  of  atoms,  C6H5-CH=, 
which  is  analogous  to  ethylidene,  CH3-CH=  (part  i.  p.  139). 


408  AROMATIC   ALCOHOLS,    ETC. 

They  do  not  reduce  Fehling's  solution,  and  they  do  not 
undergo  polymerisation  ;  they  do  not  form  additive  com- 
pounds with  ammonia,  but  yield  complex  products  such  as 
hydrobenzamide,  (C6H5-CH)3N2,  which  is  obtained  by  treating 
benzaldehyde  with  ammonia.  When  shaken  with  concen- 
trated potash  (or  soda),  they  yield  a  mixture  of  the  corre- 
sponding alcohol  and  acid  (compare  p.  403), 

2C6H5-CHO  +  KOH  =  C6H5.CH2.OH  +  C6H6.COOK. 


Nitrobenzaldehydes,  C6H4(NO2)-CHO.—  When  treated  with  a 
mjxture  of  nitric  and  sulphuric  acids,  benzaldehyde  yields  m-nitro- 
benzaldehyde  (m.p.  58°)  as  principal  product,  small  quantities  of 
o-nitrobenzaldehyde  (m.p.  46°)  being  formed  at  the  same  time. 

j?-Nitrobenzaldehyde  (m.p.  107°),  and  also  the  o-compound,  are 
most  conveniently  prepared  by  the  oxidation  of  the  corresponding 
nitrocinnamic  acids  (p.  432)  with  potassium  permanganate, 

C'H*<CH°CH.COOH  +  40  =  C«H«<CHO  +  2CO*  +  H*°- 

During  the  operation  the  mixture  is  shaken  with  benzene  in  order 
to  extract  the  aldehyde  as  fast  as  it  is  formed,  and  thus  remove  it 
from  the  further  action  of  the  oxidising  agent.  The  benzene 
solution  is  then  evaporated,  and  the  aldehyde  purified  by 
recrystallisation. 

The  nitrobenzaldehydes  are  colourless,  crystalline  substances, 
which  show  much  the  same  behaviour  as  benzaldehyde  itself  ;  when 
reduced  with  ferrous  sulphate  and  ammonia  they  are  converted  into 
the  corresponding  amidobenzaldehydes,  C6H4(NH2)-CHO. 

o-Nitrobenzaldehyde  is  a  particularly  interesting  substance,  as, 
when  its  solution  in  acetone  is  mixed  with  a  few  drops  of  dilute 
soda,  a  precipitate  of  indigo  gradually  forms  (Baeyer).  This  im- 
portant synthesis  of  this  vegetable  dye  may  be  represented  by  the 
equation 

2C6H4<CHO  +  2CH3.CO.CH3  = 

Indigo. 

+  2CH3.COOH  +  2H20. 

Hydroxy-aldeli  ydes. 

The  hydroxy-derivatives  of  the  aldehydes,  such  as  the 
hydroxybenzaldehydes,  C6H4(OH)-CHO,  which  contain  the 


AROMATIC   ALCOHOLS,    ETC.  409 

hydroxyl-group   united    with  the  nucleus,   combine  the  pro- 
perties of  phenols  and  aldehydes. 

They  may  be  obtained  by  the  oxidation  of  the  correspond- 
ing hydroxy-alcohols  ;  saligenin  (p.  404),  or  o-hydroxybenzyl 
alcohol,  for  example,  yields  salicylaldehyde  or  0-hydroxybenz- 
aldehyde, 


As,  however,  such  alcohols  are  not  easily  obtained,  and 
indeed  in  many  cases  have  only  been  produced  by  the 
reduction  of  the  hydroxy-aldehydes,  the  latter  are  usually 
prepared  by  heating  the  phenols  with  chloroform  in  alkaline 
solution  (Reimer's  reaction), 

C6HiEi.OH  +  CHC13  +  3KOH  -  C6H4<^0  +  3KC1  +  2H20. 

The  actual  changes  which  occur  in  carrying  out  Reimer's  reaction 
are  not  clearly  understood  ;  but  it  may  be  assumed  that,  in  the  first 
place,  the  phenol  interacts  with  the  chloroform  in  the  presence  of 
the  alkali,  yielding  an  intermediate  product  containing  halogen, 

C6H5-OH  +  CHC13  =  C6H4<  +  HC1> 


which  by  the  further  action  of  the  alkali  is  converted  into  a 
hydroxybenzaldehyde,  just  as  benzalchloride,  C6H5-CHC12,  is  trans- 
formed into  benzaldehyde  (compare  p.  406), 


As  a  rule,  the  primary  product  is  the  o-hydroxyaldehyde,  small 
quantities  of  the  corresponding  jo-compound  being  produced  at  the 
same  time. 

Salicylaldehyde,  C6H4(OH)-CHO  (o-hydroxybenzaldehydeW- 
may  be  obtained  by  oxidising  saligenin   with  chromic   acid 
(see   above),    but    it    is    usually    prepared    from   phenol   by 
Reimer's  reaction. 

Phenol  (20  grams)  is  dissolved  in  soda  (60  grams)  and  water  (120 
grams),  the  solution  heated  to  60°  in  a  flask  provided  with  a  reflux 
condenser,  and  chloroform  (30  grams)  added  in  small  quantities  at 
a  time  from  a  dropping  funnel.  After  slowly  heating  to  boiling, 
the  unchanged  chloroform  is  distilled  off,  the  alkaline  liquid  acidi- 


410  AROMATIC    ALCOHOLS,    ETC. 

fied  and  distilled  in  steam,  when  a  mixture  of  phenol  and  salicyl- 
aldehyde  passes  over.  (The  residue  in  the  flask  contains  j>-hydroxy- 
benzaldehyde,  which  may  be  extracted  from  the  filtered  liquid  with 
ether,  and  purified  by  recrystallisation.)  The  oily  mixture  is  ex- 
tracted from  the  distillate  with  ether,  and  the  extract  shaken  with 
dilute  sodium  bisulphite,  which  dissolves  the  aldehyde  in  the  form 
of  its  bisulphite  compound.  The  aqueous  liquid  is  then  separated, 
acidified,  and  the  regenerated  salicylaldehyde  extracted  with  ether 
and  purified  by  distillation. 

Salicylaldehyde  is  a  colourless  oil  which  boils  at  196°, 
and  possesses  a  penetrating,  aromatic  odour ;  it  is  moderately 
soluble  in  water,  its  solution  giving  a  deep  violet  colouration 
on  the  addition  of  ferric  chloride.  When  reduced  with 
sodium  amalgam,  it  yields  saligenin,  C6H4(OH)-CH2-OH 
(p.  404),  whereas  oxidising  agents  convert  it  into  salicylic 
acid,  C6H4(OH).COOH. 

p-Hydroxybenzaldehyde  is  crystalline,  and  melts  at  116°;  it 
dissolves  readily  in  hot  water,  and  gives,  with  ferric  chloride,  a 
violet  colouration. 

m-Hydroxybenzaldehyde  is  obtained  from  m-nitrobenzaldehyde 
by  conversion  into  w-amidobenzaldehyde,  and  subsequent  displace- 
ment of  the  amido-group  by  hydroxyl,  by  means  of  the  diazo- 
reaction  (p.  372).  It  crystallises  from  water  in  colourless  needles, 
and  melts  at  104°. 

Anisaldehyde,  C6H4(OCH3)-CHO  (^-methoxybenzaldehyde), 
is  prepared  from  oil  of  aniseed.  This  ethereal  oil  contains 
anethole,  C6H4(OCH3).CH:CH-CH3,  a  crystalline  substance 
which  melts  at  21°  and  distils  at  232°,  and  which  on  oxida- 
tion with  potassium  bichromate  and  sulphuric  acid  is  con- 
verted into  anisaldehyde,  the  propenyl  group  -CH:CH-CH3 
being  oxidised  to  the  aldehyde  group.  Synthetically,  it  may 
be  prepared  by  digesting  p-hydroxybenzaldehyde  with  alco- 
holic potash  and  methyl  iodide, 

°A<CHO  +  CH^  =  CA<CHOS  +  KT- 

Anisaldehyde  is  a  colourless  oil  which  boils  at  248°,  and 
possesses  a  penetrating,  aromatic  odour;  on  reduction  with 
sodium  amalgam,  it  yields  anisyl  alcohol,  C6H4(OCH3).CH2.OH 


AROMATIC   ALCOHOLS,    ETC.  411 

(p.    404)  ;    oxidising   agents    convert    it    into    anisic    acid, 
C6H4(OCH3).COOH  (p.  439). 


Ketones. 

The  ketones  of  the  aromatic,  like  those  of  the  fatty  series, 
have  the  general  formula  R  -  CO  —  R',  where  R  and  R/  re- 
present different  or  identical  radicles,  one  of  which  must,  of 
course,  be  aromatic. 

Acetophenone,  phenylmethyl  ketone,  or  acetylbenzene, 
C6H5-CO-CH3,  may  be  described  as  a  typical  aromatic  ketone. 
It  is  formed  on  distilling  a  mixture  of  calcium  benzoate  and 
calcium  acetate,  a  reaction  which  is  exactly  analogous  to  that 
which  is  made  use  of  in  obtaining  mixed  ketones  of  the  fatty 
series, 

(C6H6.COO),Ca  +  (CHs.COO)0Ca  = 

2C6H5.CO.CH3  +  2CaC03. 

It  may  also  be  obtained  by  treating  benzoyl  chloride  (p.  420) 
with  zinc  methyl,  just  as  diethyl  ketone  may  be  produced 
from  propionyl  chloride  and  zinc  ethyl  (part  i.  p.  136), 

C6H5.COC1  +  Zn(CH3)2  =  C6 


C6H5.CO-CH3  +  CH4  +  ;Zn(OH)2  +  HCli 

It  is,  however,  most  conveniently  prepared  by  treating 
benzene  with  acetyl  chloride  in  presence  of  aluminium 
chloride, 

C6H6  +  CII8.COC1  =  C6H5-CO.CH3  +  HCL 

This  method  is  of  general  use,  as  by  employing  other  acid 
chlorides  and  other  hydrocarbons,  many  other  ketones  may  be 
prepared ;  it  is  comparable  to  Friedel  and  Craft's  method  of 
preparing  hydrocarbons  (p.  329). 


412  AROMATIC   ALCOHOLS,    ETC. 

Acetophenone  is  a  crystalline  substance,  melting  at  20-5°, 
and  boiling  at  202°;  it  is  used  as  a  hypnotic  in  medicine, 
under  the  name  of  hypnone.  Its  chemical  behaviour  is 
so  similar  to  that  of  the  fatty  ketones,  that  most  of  its 
reactions,  or  at  any  rate  those  which  are  determined  by 
the  carbonyl-group,  might  be  foretold  from  a  considera- 
tion of  those  of  acetone ;  on  reduction  with  sodium 
amalgam,  acetophenone  is  converted  into  phenylmethyl 
carbinol,  C6H5-CH(OH)-CH3,  just  as  acetone  is  transformed 
into  isopropyl  alcohol ;  like  acetone,  and  other  fatty  ketones, 
it  interacts  readily  with  hydroxylamine  and  with  phenyl- 
hydrazine,  giving  the  oxime,  C6H5-C(NOH)-CH3,  and  the 
hydrazone,  C6H5-C(N2HC6H5).CH3,  respectively.  On  oxida- 
tion, it  is  resolved  into  benzoic  acid  and  carbon  dioxide, 
just  as  acetone  is  oxidised  to  acetic  acid  and  carbon 
dioxide, 

C6H5.CO-CH3  +  40  =  C6H5-COOH  +  C02  +  H20. 

Acetophenone  shows  also  the  general  behaviour  of  aromatic 
compounds,  inasmuch  as  it  may  be  converted  into  nitro-, 
amido-,  and  halogen-derivatives  by  displacement  of  hydrogen 
of  the  nucleus. 

The  homologues  of  acetophenone,  such  as  propiophenone, 
C6H5.COC2H5,  butyrophenone,  C6H5-CO.C3Hr,  &c.,  are  of 
little  importance,  but  benzophenone,  an  aromatic  ketone  of  a 
different  series,  may  be  briefly  described. 

Benzophenone,  diphenyl  ketone,  or  benzoylbenzene, 
C6H5-CO-C6H5,  may  be  obtained  by  distilling  calcium  ben- 
zoate,  and  by  treating  benzene  with  benzoyl  chloride  in 
presence  of  aluminium  chloride ;  it  is  most  conveniently 
prepared  by  adding  aluminium  chloride  to  a  solution  of 
carbonyl  chloride  in  benzene, 

2C6H6  +  COC12  =  C6H5.CO-C6H5  +  2HC1. 

It  is  a  crystalline  substance,  melting  at  48-49°,  and  is  very 
similar  to  acetophenone  in  most  respects ;  when  distilled  ovej 


AROMATIC    ALCOHOLS,    ETC.  413 

zinc-dust,  it  is  converted  into  diphenylmethane,  C6H5-CH2-C6H5 
(p.  340). 

Quinones. 

When  an  aqueous  solution  of  hydroquinone  is  oxidised 
with  excess  of  ferric  chloride,  a  dark-brown  solution  is 
obtained  which  has  a  very  penetrating  odour,  and  from 
which,  on  standing,  yellowish-brown  crystals  are  deposited, 

CGH4(OH)2  +  0  =  C6H402  +  H20.^  < 

The  substance  formed  in  this  way  is  named  quinone,  or 
benzoquinone,  and  is  the  simplest  member  of  a  very  interest- 
ing class  of  compounds. 

Quinone,  or  benzoquinone,  C6H402,  is  usually  prepared  by 
oxidising  aniline  with  potassium  bichromate  and  sulphuric 
acid. 

Aniline  (1  part)  is  dissolved  in  water  (25  parts)  and  sulphuric 
acid  (8  parts),  and  finely-powdered  potassium  bichromate  (3-5 
parts)  gradually  added,  the  whole  being  well  cooled  during  the 
operation ;  the  product,  which  is  very  dark  coloured,  owing  to 
the  presence  of  aniline  black,  is  extracted  with  ether,  the  ether 
evaporated,  and  the  crude  quinone  purified  by  recrystallisation 
from  light  petroleum  or  by  sublimation. 

Quinone  crystallises  in  golden-yellow  prisms,  melts  at  116°, 
sublimes  very  readily,  and  is  volatile  in  steam ;  it  has  a 
peculiar,  irritating,  and  very  characteristic  smell,  and  is  only 
sparingly  soluble  in  water,  but  dissolves  freely  in  alcohol 
and  ether.  It  is  readily  reduced  by  sulphurous  acid,  zinc 
and  hydrochloric  acid,  &c.,  being  converted  into  hydro- 
quinone, 

C6H402  +  2H  =  C6H4(OH)2. 

In  some  respects  quinone  behaves  as  if  it  contained  two 
carbonyl-groups,  each  having  properties  similar  to  those 
of  the  carbonyl-groups  in  compounds  such  as  acetone, 
acetophenone,  &c.  ;  when  treated  with  hydroxylamine 


414  AROMATIC   ALCOHOLS,    ETC. 

hydrochloride,  for  example,  quinone  yields  a  monoxime, 
C6H4<^^  QTT  (identical  with  nitrosophenol,  p.  367),  and  also  a 

dioxime,  C6H4<^™-  QTT-     The  two  carbonyl-groups,  moreover, 

are  in  the  para-position  to  one  another,  as  is  shown  by  the 
fact  that,  when  quinone-dioxime  is  reduced  with  tin  and 
hydrochloric  acid,  it  yields  jp-phenylenediamine. 

In  other  respects,  however,  quinone  undergoes  changes 
which  are  quite  different  from  those  observed  in  the  case  of 
ordinary  ketones ;  on  reduction,  for  instance,  each  >CO 
group  is  transformed  into  ^C-OH,  and  not  into  ^>CH-OH, 
as  might  have  been  expected  from  analogy ;  again,  on  treat- 
ment with  phosphorus  pentachloride,  each  oxygen  atom 
is  displaced  by  one  atom  of  chlorine,  £>-dichlorobenzene, 

C6H4<^Qj,  being   formed,   and   not  a  tetrachloro-derivative, 

<d 
pA  as  might  have  been  expected. 

This  curious  behaviour,  and  the  close  connection  between 
quinone  and  hydroquinone,  is  well  explained  by  assuming  that 
quinone  has  the  constitution  represented  by  the  formula  i., 
and  that  when  it  is  reduced  to  hydroquinone  (formula  n.), 

CO  C-OH 

the  two   /\   groups  are  converted  into  two  /\\   groups, 


o  OH 

I.  Quinone.  II.  Hydroquinone. 

Such  a  change  would  indeed  be  similar  to  the  formation  of 
pinacone  from  acetone,    as   in    the    latter   case   the    acetone 


A 

C 


H3  is  probably  first  reduced  to  CH3/1\CH3J  two  mole- 


AROMATIC    ALCOHOLS,    ETC. 


415 


cules  of  which  immediately  combine  to  form  pinacone  (com- 
pare part  i.  p.  138): 


OH 


CH3 
CH3 


OH 

i 


CH3 
CH3 


0 

in 


Three  other  constitutional  formulae  may  be  put  forward,  as  pos- 
sibly representing  the  constitution  of  quinone — namely  : 


The  first  of  these  is  practically  identical  with  that  given  above, 
but  the  second  and  third  are  different  and  not  so  probable,  because, 
although  tfcey  explain  in  a  simple  way  many  of  the  reactions  of 
quinone,  they  do  not  so  readily  account  for  the  formation  of  a 
dioxime. 

Benzoquinone  and  many  other  para-quinones  (that  is  to  say, 
quinones  in  which  the  two  carbonyl-groups  are  in  the  para- 
position  to  one  another"*)  may  be  produced  by  the  oxidation, 
with  chromic  acid  or  ferric  chloride,  of  many  hydroxy-  and 
amido-compounds,  which  contain  the  substituting  groups  in 
the  para-position ;  quinone,  for  example,  is  formed  on 
oxidising  ^>-amidophenol,  C6H4(OH)-NH2,  and  ^-phenylenedi- 
amine,  C6H4(NH2)2,  whereas  o-toluidine,  ^-toluylenediamine, 
C6H4(NH2)2.CH3,  [NH2:NH2:CH3  =  1:4:6],  &c.,  yield  tolu- 
quinone.  o 


*  Other  quinones,  of  a  somewhat  different  class  to  benzoquinone,  are 
described  later  (pp.  456,  470). 


416  AROMATIC    ALCOHOLS,    ETC. 

When,  however,  bleaching-powder  is  used  as  the  oxidising  agent, 
quinone  chlorimides  and  quinone  dichlorodiimides  are  formed  in  the 
place  of  quinone, 

NH2.C6H4-OH  +  4C1  =  NC1:C6H4:0  +  3HC1 

Quinone  Chlorimide. 

NH2-C6H4.NH2  +  6C1  =  NC1:C6H4:NC1  +  4HC1. 

Quinone  Dichlorodiimide. 

The  quinone  chlorimides  and  dichlorodiimides  resemble  quinone  in 
many  respects ;  they  are  crystalline,  readily  volatile  in  steam,  and 
are  respectively  converted  into  ^-amidophenol  and  #?-phenylenedi- 
amine  or  their  derivatives  on  reduction. 

Chloranil,  or  tetrachloroquinone,  O:C6C14:O,  is  produced  when 
chlorine  acts  on  quinone,  but  it  is  usually  prepared  by  treating 
phenol  with  hydrochloric  acid  and  potassium  chlorate,  oxidation 
and  chlorination  taking  place  simultaneously, 

C6H5-OH  +  10C1  +  O  =  O:C6C14:O  +  6HC1. 

It  crystallises  in  yellow  plates,  sublimes  without  melting,  and  is 
sparingly  soluble  in  alcohol,  and  insoluble  in  water. 

It  is  readily  reduced  to  tetrachlorohydroquirione,  OH-C6C14-OH, 
and  is  therefore  a  powerful  oxidising  agent,  for  which  reason  it  is 
much  employed  in  colour  chemistry,  when  the  use  of  inorganic 
oxidising  agents  is  undesirable. 


CHAPTEK    XXVIII. 

CARBOXYLIC    ACIDS. 

The  carboxylic  acids  of  the  aromatic  series  are  derived  from 
the  aromatic  hydrocarbons,  just  as  those  of  the  fatty  series  are 
derived  from  the  paraffins — namely,  by  the  substitution  of  one 
or  more  carboxyl-groups  for  a  corresponding  number  of 
hydrogen  atoms.  In  this,  as  in  other  cases,  however,  one  of 
two  classes  of  compounds  may  he  obtained  according  as 
substitution  takes  place  in  the  nucleus  or  in  the  side-chain ; 
benzene  yields,  of  course,  only  acids  of  the  first  class,  such  as 
benzoic  acid,  C6H5-COOH,  the  three  (o.m.p.)  phthalic  acids, 
C6H4(COOH)2,  the  three  tricarboxylic  acids,  C6H3(COOH)3, 
&c.,  but  toluene  and  all  the  higher  homologues  may  give 


CARBOXYLIC    ACIDS.  417 

rise    to    derivatives   of    both    kinds—  as,    for   example,   the 
three  toluic  acids,  C6H4(CH3)-COOH,  and  phenylacetic  acid, 


Although  there  are  no  very  important  differences  in  the 
properties  of  these  two  classes  of  acids,  it  is  more  convenient 
to  describe  them  separately,  taking  first  those  compounds  in 
which  the  carboxyl-groups  are  directly  united  with  carbon  of 
the  nucleus. 

Preparation.  —  Such  acids  may  be  obtained  by  oxidising  the 
alcohols  or  aldehydes, 

C6H5-CH2-OH  +  20  =  C6H5.COOH  +  H20 

C6H5-CHO  +  0  =  C6H5.COOH, 

and  by   hydrolysing  the  nitriles  (p.    421)   with  alkalies  or 
mineral  acids, 

C6H5-C2sT  +  2H00  =  C6H5-COOH  +  NH3 
C6H5.CH2-CN  +  2H20  =  C6H5-CH2.COOH  +  NH3, 

reactions  which  are  exactly  similar  to  those  employed  in  the 
case  of  the  fatty  acids  (part  i.  p.  165). 

Perhaps,  however,  the  most  important  method,  and  one 
which  has  no  counterpart  in  the  fatty  series,  consists  in  oxidis- 
ing the  homologues  of  benzene  with  dilute  nitric  acid  or 
chromic  acid, 

C6H5-CH3  +  30  =  C6H5-COOH  +  H20 
C6HB.CH2.CH8  +  60  -  C6H5-COOH  +  C02  +  H20. 
In  this  way  only  those  acids  which  contain  the  carloxyl-group 
united  with  the  nucleus  can  be  obtained,  because  the  side-chain 
is  always  oxidised  to  -COOH,  no  matter  how  many  -CH2- 
groups  it  may  contain  ;  in  other  words,  all  homologues  of 
benzene  which  contain  only  one  side-chain  yield  benzoic  acid, 
whereas  those  containing  two  give  one  of  the  phthalic  acids. 
In  the  latter  case,  however,  one  of  the  side-chains  is  oxidised 
before  the  other  is  attacked,  so  that  by  stopping  the  process  at 
the  right  time,  an  alkyl-derivative  of  benzoic  acid  is  obtained, 

C6H4(CH3)2  +  30  =  C6H4(CH3).COOH  +  H20 

C6H4(CH3).COOH  +  30  -  C6H4(COOH),  +  H20. 

2  A 


418  CARBOXYLIC   ACIDS. 

Oxidation  is  frequently  carried  out  by  boiling  the  hydrocarbon 
(1  vol.)  with  nitric  acid  (1  vol.)  diluted  with  water  (2-4  vols.)  until 
brown  fumes  are  no  longer  formed.  The  mixture  is  then  made 
slightly  alkaline  with  soda,  and  any  unchanged  hydrocarbon  and 
traces  of  nitro-hydrocarbon  separated  with  a  funnel  or  extracted 
with  ether  ;  the  alkaline  solution  is  then  acidified  and  the  acid 
separated  by  filtration  or  extracted  with  ether,  and  purified  by 
recrystallisation. 

Most  hydrocarbons  are  only  very  slowly  attacked  by  dilute  nitric 
or  chromic  acid  ;  in  such  cases  it  is  advantageous  to  first  substitute 
chlorine  or  some  other  group  for  hydrogen  of  the  side-chain,  as  in 
this  way  oxidation  is  facilitated.  Benzyl  chloride,  C6H5-CH2C1,  for 
example,  is  much  more  readily  oxidised  than  toluene,  wlfereas 
benzyl  acetate,  C6H5-CH2-OC2H30  (p.  349),  and  benzyl  ethyl  ether, 
C6H5-CH2-O-C2H5,  are  even  more  readily  attacked. 

. 

Properties. — The  aromatic  acids  are  crystalline,  and  distil 

without  decomposition ;  they  are  sparingly  soluble  in  cold 
water,  but  much  more  readily  in  hot  water,  alcohol,  and  ether. 
As  regards  all  those  properties  which  are  determined  by  the 
carboxyl-group,  the  aromatic  acids  are  closely  analogous  to  the 
fatty  compounds,  and  give  corresponding  derivatives,  as  the 
following  examples  show  : 

Benzoicacid,        C6H5.COOH        Benzoyl  chloride,    C6H5-COC1. 
Sodium  benzoate,C6H5-COONa      Benzamide,  C6H5-CO-NH2. 

Ethyl  benzoate,  C6H5-COOC2H5  Benzoic  anhydride,  (C6H5-CO)2O. 

When  distilled  with  lime,  they  are  decomposed  with  loss  of 
carbon  dioxide  and  formation  of  the  corresponding  hydro- 
carbons, just  as  acetic  acid  under  similar  circumstances  yields 
marsh-gas, 

C6H5-COOH  =  C6H6  +  C02 
C6H4(CH3).COOH  =  C6H6-CH3  +  C02. 

Benzoic  acid,  C6H5-COOH,  occurs  in  the  free  state  in 
many  resins,  especially  in  gum  benzoin  and  Peru  balsam ;  also 
in  the  urine  of  cows  and  horses,  as  hippuric  acid  or  benzoyl- 
glycine,  C6H5-CO-NH.CH2.COOH,  to  the  extent  of  about 
two  per  cent. 

It  is  generally  prepared  either  by  the  sublimation  of  gum 


CARBOXYLIC   ACIDS.  419 

benzoin  in  iron  pots,  the  crude  sublimate  being  purified  by 
recry stall! sation  from  water,  or  by  treating  hippuric  acid 
with  hydrochloric  acid  (part  i.  p.  292), 

CeH5-CO.NH.CHg.COOH  +  HC1  +  H20  = 

C6H5-COOH  +  NH2.CH2.COOH,  HC1. 

Glycine  Hydrochloritle. 

The  urine  of  horses,  cows,  or  other  herbivorous  animals  is  evapor- 
ated to  one-third  of  its  volume,  filtered,  and  acidified  with  hydro- 
chloric acid ;  the  crystals  of  hippuric  acid  which  are  deposited  on 
standing,  are  collected  and  boiled  for  a  short  time  with  four  parts 
of  concentrated  hydrochloric  acid,  the  benzole  acid  which  separates 
on  cooling  being  purified  by  recrystallisation  ;  the  mother-liquors 
contain  glycine  hydrochloride. 

Benzoic  acid  is  manufactured  by  oxidising  benzyl  chloride 
(p.  348)  with  60  per  cent,  nitric  acid, 

C6H5.CH2C1  +  20  =  C6H5-COOH  +  HC1. 

It  may  also  be  prepared  by  oxidising  toluene,  or  by  any  other 
of  the  general  methods. 

Benzoic  acid  separates  from  water  in  glistening  crystals, 
melts  at  120°,  and  boils  at  250°,  but  it  sublimes  very  readily 
even  at  100°,  and  is  volatile  in  steam  ;  it  dissolves  in  400 
parts  of  water  at  15°,  but  is  readily  soluble  in  hot  water, 
alcohol,  and  ether.  Its  vapour  has  a  characteristic  odour,  and 
an  irritating  action  on  the  throat,  causing  violent  coughing. 
Most  of  the  metallic  salts  of  benzoic  acid  are  soluble  in  water 
and  crystallise  well;  calcium  benzoate,  (C6H5-COO)2Ca  +  3H2O, 
for  example,  prepared  by  neutralising  benzoic  acid  with  milk 
of  lime,  crystallises  in  needles,  and  is  very  soluble  in  water. 

The  ethereal  salts  are  prepared  in  precisely  the  same  way 
as  those  of  the  fatty  acids  (part  i.  p.  187);  ethyl  benzoate, 
for  example,  C^Hg-COOCgHJ,  is  obtained  by  saturating  an 
alcoholic  solution  of  benzoic  acid  with  hydrogen  chloride,  and 
after  some  time  pouring  the  solution  into  water,  the  pre- 
cipitated oil  being  purified  by  fractional  distillation.  It  boils 
at  211°,  has  a  pleasant  aromatic  odour,  and  is  readily  hydro- 
lysed  by  boiling  alcoholic  potash, 


420  CARBOXYLIC   ACIDS. 

Benzoyl  chloride,  C6H5-COC1,  is  obtained  by  treating 
benzoic  acid  with  phosphorus  pentachloride.  It  is  a  colourless 
oil,  possessing  a  very  irritating  odour,  and  boils  at  200° ;  it  is 
gradually  decomposed  by  water,  yielding  benzoic  acid  and 
hydrochloric  acid. 

Benzoic  anhydride,  (C6H5-CO)20,  is  produced  when  benzoyl 
chloride  is  treated  with  sodium  benzoate,  just  as  acetic  anhy- 
dride is  formed  by  the  interaction  of  acetyl  chloride  and 
sodium  acetate  (part  i.  p.  1 60) ;  it  is  a  crystalline  substance, 
melting  at  42°,  and  closely  resembles  acetic  anhydride  in 
ordinary  chemical  properties. 

Benzoyl  chloride  and  benzoic  anhydride  may  be  used  for 
the  detection  of  hydroxy-compounds,  as  they  interact  with  all 
such  substances  (although  not  so  readily  as  the  corresponding 
derivatives  of  acetic  acid,  part  i.  p.  159),  yielding  benzoyl- 
derivatives,  the  monovalent  benzoyl-grouip,  C6H5-CO-,  taking 
the  place  of  the  hydrogen  of  the  hydroxyl-group, 

C6H5-OH  +  C6H5-COC1  -  C6H5.O.CO.C6H5  +  HC1 

Phenyl  Benzoate. 

C2H5-OH  +  (C6H5-CO)20  -  C2H5.O.CO-C6H5  +  C6H5-COOH. 

Ethyl  Benzoate. 

Benzoyl -derivatives  may  be  prepared  by  heating  the  hydroxy- 
compovmd  with  benzoyl  chloride  or  with  benzoic  anhydride.  A 
more  convenient  method,  however,  and  one  which  gives  a  purer 
product,  is  that  of  Baumann  and  Schotten  :  it  consists  in  adding 
benzoyl  chloride  and  10  per  cent,  potash  alternately,  in  small 
quantities  at  a  time,  to  the  hydroxy- com  pound,  which  is  either 
dissolved  or  suspended  in  water,  the  mixture  being  well  shaken 
and  kept  cool  during  the  operation.  Potash  alone  is  then  added 
until  the  disagreeable  smell  of  benzoyl  chloride  is  no  longer 
noticed,  and  the  product  finally  separated  by  filtration  or  by 
extraction  with  ether.  This  method  is  also  used  in  preparing 
benzoyl-derivatives  of  amido-compounds ;  aniline,  for  example, 
yields  benzoyl-aniline, 

C6H5-NH2  +  C6H5.COC1  =  C6H5-NH.CO.C6H5  +  HC1. 

In  the  above  method  the  alkali  serves  to  neutralise  the  hydrochloric 
acid  as  fast  as  it  is  formed,  the  interaction  taking  place  much 
more  readily  in  the  neutral  or  slightly  alkaline  solution. 


CARBOXYLIC   ACIDS.  421 

Benzamide,  C6H5-CO-NH2,  may  be  taken  as  an  example  of 
an  aromatic  amide ;  it  may  be  obtained  by  reactions  similar 
to  those  employed  in  the  case  of  acetamide  (part  i.  p  162),  as, 
for  example,  by  treating  ethyl  benzoate  with  ammonia, 

C6H5.COOC2H5  +  NHg  -  C6H6.CO.NH2  +  C2H5-OH; 

but  it  is  most  conveniently  prepared  by  triturating  benzoyl 
chloride  with  dry  ammonium  carbonate  in  a  mortar,  and 
purifying  the  product  by  recrystallisation  from  water, 

2C6H5.COC1  +  (NH4)2C08  = 

2C6H5.CONH2  +  C02  +  H20  +  2HC1. 

It  is  a  colourless,  crystalline  substance,  melts  at  130°,  and  is 
sparingly  soluble  in  cold,  but  readily  soluble  in  hot,  water ; 
like  other  amides,  it  is  decomposed  by  boiling  alkalies,  yield- 
ing ammonia  and  an  alkali  salt, 

C6H5.CO-NH2  +  KOH  -  C6H6.COOK  +  NH3. 
Benzonitrile,  or  phenyl  cyanide,  C6H5-CN,  may  be  obtained 
by   treating   benzamide   with  dehydrating  agents,  a  method 
similar  to  that  employed  in  the  preparation  of  fatty  nitriles, 

C6H5.CO-NH2  =  C6H6-CN  +  H20. 

Although  it  cannot  be  prepared  by  treating  chloro-  or  bromo- 
benzene  with  potassium  cyanide  (the  halogen  atom  being  so 
firmly  held  that  no  interaction  occurs),  it  may  be  obtained  by 
fusing  benzenesulphonic  acid  with  potassium  cyanide  (or  with 
potassium  ferrocyanide,  which  yields  the  cyanide),  just  as 
fatty  nitriles  may  be  prepared  by  heating  the  alkylsulphuric 
acids  with  potassium  cyanide, 

C6H5.S08K  +  KCN  =  C6H5-CN  +  K2S03 
C2H5-S04K  +  KCN  =  C2H5-CN  +  K2S04. 

It  is,  however,  most  conveniently  prepared  from  aniline  by 
Sandmeyer's  reaction — namely,  by  treating  a  solution  of  diazo- 
benzene  chloride  with  potassium  cyanide  and  copper  sulphate 
(P-  372), 

C6H6-N2C1  +  KCN  =  C6H6-ClSr  +  KC1  +  N2. 


422  OARBOXYLIC   ACIDS. 

Benzonitrile  is  a  colourless  oil,  boiling  at  191°,  and  smells  like 
nitrobenzene.  It  undergoes  changes  exactly  similar  to  tbose 
which  are  characteristic  of  fatty  nitriles,  being  converted  into  the 
corresponding  acid  on  hydrolysis  with  alkalies  or  mineral  acids, 

C6H5-CK  +  2H20  =  C6H5-COOH  +  NH3, 
and  into  a  primary  amine  on  reduction, 

C6H5.ON  +  4H  =  C6H5.CH2-NH2. 

Benzylamine. 

Other  aromatic  nitriles,  such  as  the  three  tolunitriles, 
C6H4(CH3)-CN,  are  known,  also  compounds  such  as  phenyl- 
acetonitrile  (benzyl  cyanide,  p.  429),  C6H5-CH2-CN,  which 
contain  the  cyanogen  group  in  the  side-chain. 

Substitution  Products  of  Benzoic  Acid. — Benzoic  acid  is 
attacked  by  halogens  (although  not  so  readily  as  the  hydro- 
carbons), the  product  consisting  of  the  ?ftefa-derivative  (p.  351); 
when,  for  example,  benzoic  acid  is  heated  with  bromine  and 
water  at  125°,  m-bromobenzoic  acid,  C6H4Br-COOH  (m.p. 
1 55°),  is  formed.  The  o-  and ^>-bromobenzoic  acids  are  obtained 
by  oxidising  the  corresponding  bromotoluenes  with  nitric 
acid;  the  former  melts  at  148°,  the  latter  at  251°.  Nitric 
acid,  in  the  presence  of  sulphuric  -acid,  acts  readily  on  benzoic 
acid,  ?n-nitrobenzoic  acid,  C6H4(N02)-COOH  (m.p.  142°), 
being  the  principal  product;  o-nitrobenzoic  acid  (m.p.  147°) 
and  ^9-nitrobenzoic  acid  (m.p.  240°)  are  obtained  by  the  oxida- 
tion of  o-  and  p-nitrotoluene  respectively  (p.  355) ;  when  these 
acids  are  reduced  with  tin  and  hydrochloric  acid,  they  yield 
the  corresponding  amidobenzoic  acids,  C6H4(NH2)-COOH, 
which,  like  glycine  (part  i.  p.  292),  form  salts  both  with 
acids  and  bases. 

When  heated  with  sulphuric  acid,  benzoic  acid  is  converted  into 
w-sulphobenzoic  acid,  C6H4(S03H)-COOH,  small  quantities  of  the 
^-acid  also  being  produced.  The  o-acid  is  obtained  by  oxidising 
toluene-o-sulphonic  acid  ;  when  treated  with  ammonia  it  yields  an 
imide  (p.  426), 

+  2H30, 


CARBOXYLIC   ACIDS.  423 

which  is  remarkable  for  possessing  an  exceedingly  sweet  taste,  and 
which  conies  into  the  market  under  the  name  of  saccharin. 

The  sulphobenzoic  acids  are  very  soluble  in  water  ;  when  fused 
with  potash  they  yield  hydroxy-acids  (p.  433),  just  as  benzene- 
sulphonic  acid  gives  phenol, 

C6H4(S03K).COOK  +  2KOH  =  C6H4(OK).COOK  +  K2S03  +  H2O. 

The  three  (o.m.p.)  toiuic  acids,  C6H4(CH3)-COOH,  may 
be  produced  by  oxidising  the  corresponding  xylenes  with 
dilute  nitric  acid, 

C6H4(CH3)2  +  30  =  C6H4(CH3).COOH  +  H20, 

but  the  o-  and  ^?-acids  are  best  prepared  by  converting  the 
corresponding  toluidines  into  the  nitriles  by  Sandmeyer's 
reaction  (p.  372),  and  then  hydrolysing  with  acids  or  alkalies, 


As  m-toltiidine  cannot  easily  be  obtained,  and  as  w-xylene  is 
only  very  slowly  oxidised  by  dilute  nitric  acid,  in  order  to  pre- 
pare ??i-toluic  acid,  ?w-xylyl  bromide,  C6H4(CH3)-CH2Br  (b.p. 
215°),  is  first  prepared  by  adding  bromine  (1  mol.)  to  boiling 
w-xylene  (1  mol.)  ;  this  product  is  then  heated  with  sodium 
ethoxide,  in  alcoholic  solution,  to  convert  it  into  m-xylyl  ethyl 
ether,  C6H4(CH3).CH2.0-C2H5  (b.p.  204°),  a  substance  which 
is  readily  oxidised  by  potassium  bichromate  and  sulphuric  acid 
(p.  418),  yielding  m-toluic  acid.  The  three  o-,  ra-,  p-toluic 
acids  melt  at  103°,  110°,  and  180°  respectively,  and  resemble 
benzoic  acid  very  closely,  but  since  they  contain  a  methyl- 
group,  they  have  also  properties  which  are  not  shown  by 
benzoic  acid  ;  on  oxidation,  for  example,  they  are  converted 
into  the  corresponding  phthalic  acids,  just  as  toluene  is  trans- 
formed into  benzoic  acid, 

30  =  CH 


Dibasic  Acids. 
The   most   important   dicarboxylic    acids    are    the    three 


424  CABBOXYLIC    ACIDS. 

(o.m.p.)  phthalic  acids,   or  benzenedicarboxylic  acids,   which 
are  represented  by  the  formulae, 

COOH  COOH 

\         ^iCOOH 


COOH  ^^^  COOH 

COOH 
Phthalic  Acid.  Isophthalic  Acid.  Terephthalic  Acid. 

These  compounds  may  be  prepared  by  the  oxidation  of  the 
corresponding  dimethylbenzenes  with  dilute  nitric  acid,  or 
more  conveniently  by  treating  the  toluic  acids  with  potassium 
permanganate  in  alkaline  solution, 

Cecils  +  60  =  cA<cool  +  2H*° 
30  -  < 


They  are  colourless,  crystalline  substances,  and  have  all  the 
ordinary  properties  of  carboxylic  acids.  They  yield  neutral 
and  acid  metallic  salts,  ethereal  salts,  acid  chlorides,  amides, 
&c.,  which  are  similarly  constituted  to,  and  formed  by  the 
same  reactions  as,  those  of  other  dicarboxylic  acids  (part  i. 
pp.  234-238). 

Phthalic    acid,  like    succinic  acid  (part   i.    pp.   234-236), 
yields  an  anhydride  when  strongly  heated, 


but  it  is  very  important  to  notice  that  no  anhydride  of  iso- 
phthalic  acid  or  of  terephthalic  acid  can  be  produced ;  it  may, 
in  fact,  be  accepted  as  a  general  rule  that  anhydride  formation 
takes  place  only  when  the  two  carboxyl-groups  in  the  benzene 
nucleus  are  in  the  oposition,  never  when  they  occupy  the 
m-  or  ^-position. 


CARBOXYLIC    ACIDS.  425 

When  cautiously  heated  with  lime  (1  mol.)  the  phthalic 
acids  yield  benzoic  acid, 

+  co2, 

but  if  excess  of  lime  be  employed,  and  the  distillation  con- 
ducted at  a  high  temperature,  both  carboxyl-groups  are 
displaced  by  hydrogen,  and  benzene  is  formed, 

C6H4<coOH  =  GG^G  +  2C°2 ; 

this  behaviour  clearly  shows  that  these  acids  are  all  dicarboxy- 
derivatives  of  benzene. 

When  a  trace  of  phthalic  acid  is  heated  with  *  resorcinol 
and  a  drop  of  sulphuric  acid,  fluorescein  (p.  520)  is  produced, 
and  the  reddish-brown  product,  when  dissolved  in  dilute  soda 
and  poured  into  a  quantity  of  water,  yields  a  magnificently 
fluorescent  solution.  This  reaction  is  shown  by  all  the  o- 
dicarboxylic  acids  of  the  benzene  series,  but  not  by  the  m- 
and  ^9-dicarboxylic  acids. 

Phthalic  acid,  C6H4(COOH)2  (benzene-o-dicarboxylic  acid), 
may  be  obtained  by  oxidising  o-xylene  or  o-toluic  acid,  but 
it  is  usually  manufactured  by  the  oxidation  of  naphthalene 
(p.  442)  with  chromic  acid ;  for  laboratory  purposes  naphtha- 
lene tetrachloride,  C10H8C14  (p.  450),  is  oxidised  with  nitric 
acid. 

Concentrated  nitric  acid  (sp.  gr.  T45,  10  parts)  is  gradually 
added  to  naphthalene  tetrachloride  (1  part),  and  the  mixture  heated 
until  a  clear  solution  is  produced.  This  is  then  evaporated  to  dry- 
ness,  and  the  residue  distilled,  the  phthalic  anhydride  (see  below), 
which  passes  over,  being  reconverted  into  phthalic  acid  by  dissolving 
it  in  dilute  soda  ;  the  acid  is  then  precipitated  by  adding  a  mineral 
acid,  and  the  crystalline  precipitate  purified  by  recrystallisation 
from  water. 

Phthalic  acid  crystallises  in  colourless  prisms,  and  melts  at 
184°,  with  formation  of  the  anhydride,  so  that,  if  the  melted 
substance  be  allowed  to  solidify,  and  the  melting-point  again 


426  CARBOXYLIC    ACIDS. 

determined,  it  will  be  found  to  be  about  128°,  the  melting- 
point  of  phthalic  anhydride. 

Phthalic  acid  is  readily  soluble  in  hot  water,  alcohol,  and 
ether,  and  gives  with  metallic  hydroxides  well-characterised 

salts ;  the  barium  salt,  C6H4<^pQQ^>Ba,  obtained  as  a  white 

precipitate  by  adding  barium  chloride  to  a  neutral  solution  of 
the  ammonium  salt,  is  very  sparingly  soluble  in  water. 

Ethyl  phthalate,  C6H4(COOC2H5)2,  is  readily  prepared  by 
saturating  an  alcoholic  solution  of  phthalic  acid  (or  its  anhy- 
dride) with  hydrogen  chloride.  It  is  a  colourless  liquid, 
boiling  at  295°. 

Phthalyl  chloride,  C6H4(COC1)2,  is  prepared  by  heating  phthalic 
anhydride  (1  mol.)  with  phosphorus  pentachloride  (1  mol.).  It  is  a 
colourless  oil,  which  boils  at  275°,  and  is  slowly  decomposed  by 
water,  with  regeneration  of  phthalic  acid.  In  many  of  its  reactions 
it  behaves  as  if  it  had  the  constitution  represented  by  the  formula 

C6H4<^QQ^>0  (compare  succinyl  chloride,  part  i.  p.  237). 

Phthalic   anhydride,    C6H4<^^>0,     is     formed     when 

phthalic  acid  is  distilled.  It  sublimes  readily  in  long  needles, 
melts  at  128°,  boils  at  284°,  and  is  only  very  gradually 
decomposed  by  water,  but  dissolves  readily  in  alkalies,  yielding 

salts  of  phthalic  acid.     When  heated  in  a  stream  of  ammonia 

/"i/-\ 

it  is  converted  into  phthalimide,  C6H4<CQQ/>NH,  a  sub- 
stance which  melts  at  229°,  and  yields  a  potassium  derivative, 
on  treatment  with  alcoholic  potash.  There 

is  thus  a  great  similarity  between  phthalimide  and  succini- 
mide  (part  i.  p.  237). 

Isophthalic  acid,  C6H4(COOH)2  (benzene-m-dicarboxylic 
acid),  is  produced  by  oxidising  m-xylene  or  ?w-xylyl  diethyl 
ether,  C6H4(CH2-OC2H5)2  (compare  p.  418),  with  nitric  acid 
or  chromic  acid  ;  or  from  ??i-toluic  acid  (p.  423)  by  oxidation 
with  potassium  permanganate  in  alkaline  solution. 


CARBOXYLIC   ACIDS.  427 

Tt  crystallises  in  needles,  melts  above  300°,  and  when 
strongly  heated,  sublimes  unchanged;  it  is  very  sparingly 
soluble  in  water.  Methyl  isophthalate,  C6H4(COOCH3)2, 
melts  at  65°c 

Terephthalic  acid,  C6H4(COOH)2  (benzene-^-dicarboxylic 
acid),  is  formed  by  the  oxidation  of  jp-xylene,  p-toluic  acid, 
and  of  all  di-alkyl  substitution-derivatives  of  benzene,  which, 
like  cymene,  CH3-C6H4-CH(CH3)2,  contain  the  alkyl-groups  in 
the  ^-position.  It  is  best  prepared  by  oxidising  j>-toluic  acid 
(p.  423)  in  alkaline  solution  with  potassium  permanganate. 

Terephthalic  acid  is  almost  insoluble  in  water,  and,  when 
heated,  sublimes  without  melting ;  the  methyl  salt, 
C6H4(COOCH3)2,  melts  at  140°. 

Acids,  such  as  isophthalic  acid  and  terephthalic  acid,  which  have  no 
definite  melting-point,  or  which  melt  above  300°,  are  best  identified 
by  conversion  into  their  methyl  salts,  which  generally  crystallise 
well,  and  melt  at  a  comparatively  low  temperature. 

For  this  purpose  a  centigram  of  the  acid  is  warmed  in  a  test 
tube  with  about  three  times  its  weight  of  phosphorus  pentachloride, 
and  the  clear  solution,  which  now  contains  the  chloride  of  the  acid, 
poured  into  excess  of  methyl  alcohol.  As  soon  as  the  vigorous 
reaction  has  subsided,  the  liquid  is  diluted  with  water,  the  crude 
methyl  salt  collected,  recrystallised,  and  its  melting-point  deter- 
mined. 

Phenylacetic  Acid,  Phenylpropionic  Acid,  and  their 
Derivatives. 

Many  cases  have  already  been  met  with  in  which  aromatic 
compounds  have  been  found  to  have  certain  properties  similar 
to  those  of  members  of  the  fatty  series,  and  it  has  been 
pointed  out  that  this  is  due  to  the  presence  in  the  former  of 
groups  of  atoms  (side-chains)  which  may  be  considered  as 
fatty  radicles ;  benzyl  chloride,  for  example,  has  some  properties 
in  common  with  methyl  chloride,  benzyl  alcohol  with  methyl 
alcohol,  benzylamine  with  methylamine,  and  so  on,  simply 
because  similar  groups  or  radicles  in  a  similar  state  of  combin- 
ation confer,  as  a  rule,  similar  properties  on  the  compounds 


428  CARBOXYLIC   ACIDS. 

in  which  they  occur.  Inasmuch,  however,  as  nearly  all  fatty 
compounds  may  theoretically  be  converted  into  aromatic  com- 
pounds of  the  same  type  by  the  substitution  of  a  phenyl  group 
for  hydrogen,  it  follows  that  any  series  of  fatty  compounds 
may  have  its  counterpart  in  the  aromatic  group.  This  is 
well  illustrated  in  the  case  of  the  carboxylic  acids,  because, 
corresponding  with  the  fatty  acids,  there  is  a  series  of  aromatic 
acids  which  may  be  regarded  as  derived  from  them  in  the 
manner  just  mentioned : 

Formic  acid,  H-COOH, 

Benzoic  acid,  C6H5-COOH  (phenylformic  acid). 
Acetic  acid,  CH3.COOH, 

Phenylacetic  acid,  C6H5-CH2.COOH. 
Propionic  acid,  CH3.CH2.COOH, 

Phenylpropionic  acid,  C6H5.CH2-CH2.COOH. 
Butyric  acid,  CH3-CH2.CH2.COOH, 

Phenylbutyric  acid,  C6H5.CH2.CH2-CH2.COOH. 

With  the  exception  of  benzoic  acid  all  the  above  aromatic 
acids  are  derived  from  the  aromatic  hydrocarbons  by  the  sub- 
stitution of  carboxyl  for  hydrogen  of  the  side-chain.  They 
have  not  only  the  characteristic  properties  of  aromatic  com- 
pounds in  general,  but  also  those  of  fatty  acids,  and,  like  the 
latter,  they  may  be  converted  into  unsaturated  compounds  by 
loss  of  two  or  more  atoms  of  hydrogen,  giving  rise  to  new 
series,  as  the  following  example  will  show : 

Propionic  acid,  CH3.CH2-COOH, 

Phenylpropionic  acid,  C6H5-CH2.CH2.COOH 
Acrylic  acid,  CH2:CH.COOH, 

Phenylacrylic  acid,  C6H5.CH:CH-COOH. 
Propiolic  acid,  CHiC-COOH, 

Phenylpropiolic  acid,  C6H5-CiC-COOH. 

Preparation. — Aromatic  acids,  containing  the  carboxyl- 
group  in  the  side-chain,  may  be  prepared  by  carefully  oxidis- 
ing the  corresponding  alcohols  and  aldehydes,  and  by  hydro- 
lysing  the  nitriles  with  alkalies  or  mineral  acids, 

C6H5.CH2-CN  +  2H20  =  C6H5.CH2.COOH  +  NH8, 


CARBOXYLIC    ACIDS.  429 

but  these  methods  are  Hunted  in  application,  owing  to  the 
difficulty  of  obtaining  the  requisite  substances. 

The  most  important  general  methods  are :  (a)  By  the  reduc- 
tion of  the  corresponding  unsaturated  acids,  compounds  which 
are  prepared  without  much  difficulty  (p.  430), 

C6H5.CH:CH-COOH  +  2H  =  C6H5-CH2.CH2.COOH ; 

and  (b)  by  treating  the  sodium  compound  of  ethyl  malonate 
or  of  ethyl  acetoacetate  with  the  halogen  derivatives  of  the 
aromatic  hydrocarbons.  As,  in  the  latter  case,  the  pro- 
cedure is  exactly  similar  to  that  employed  in  preparing 
fatty  acids  (part  i.  pp.  189,  194,  and  198),  one  example  only 
need  be  given — namely,  the  synthesis  of  phenylpropionic 
acid. 

The  sodium  compound  of  ethyl  malonate  is  heated  with 
benzyl  chloride,  and  the  ethyl  benzylmalonate  which  is  thus 
produced, 

C6H6.CH,C1  +  CH^a(COOC2H5)2  = 

C6H5.CH2.CH(COOC2H5)2  +  NaCl, 

Ethyl  Benzylmalonate. 

is  hydrolysed  with  alcoholic  potash.  The  benzylmalonic  acid 
is  then  isolated,  and  heated  at  200°,  when  it  is  converted  into 
phenylpropionic  acid,  with  loss  of  carbon  dioxide, 

C6H5.CH2.CH(COOH)2  -  C6H5.CH2-CH2.COOH  +  C02. 

It  should  be  remembered  that  only  those  halogen  derivatives 
in  which  the  halogen  is  in  the  side-chain  can  be  employed  in 
sucli  syntheses,  because  when  the  halogen  is  united  with  the 
nucleus,  as  in  monochlorotoluene,  C6H4C1-CH3,  for  example, 
no  action  takes  place  (compare  p.  346). 

The  properties  of  two  of  the  most  typical  acids  of  this 
class  are  described  below. 

Phenylacetic  acid,  or  a-toluic  acid,  C6H5.CH2-COOH,  is  pre- 
pared by  boiling  a  solution  of  benzyl  chloride  (1  mol.)  and 
potassium  cyanide  (1  mol.)  in  dilute  alcohol  for  about  three 
hours  ;  the  benzyl  cyanide  which  is  thus  formed  is  purified 


430  CARBOXYLIG   ACIDS. 

by  fractional  distillation,  and  the  fraction  220-235°  (benzyl 
cyanide  boils  at  232°)  is  hydrolysed  by  boiling  with  dilute 
sulphuric  acid,  the  product  being  purified  by  recrystallisation 
from  water, 

C6H5.CH2C1  — >  C6H5.CH2.CN  >  C6H6.CH2.COOH. 

Phenylacetic  acid  melts  at  76-5°,  boils  at  262°,  and  crystallises 
from  boiling  water  in  glistening  plates ;  it  has  an  agreeable, 
characteristic  smell,  and  forms  salts  and  derivatives  just  as 
do  benzoic  and  acetic  acids. 

When  oxidised  with  chromic  acid  it  yields  benzoic  acid,  a 
change  very  different  to  that  undergone  by  the  isomeric  toluic 
acids  (p.  423), 

C6H5.CH2.COOH  +  30  =  C6H5-COOH  +  C02  +  H20, 

Phenylpropionic  acid,  C6H5.CH2-CH2.COOH  (hydrocinn- 
amic  acid),  is  most  conveniently  prepared  by  reducing 
cinnamic  acid  (see  below)  with  sodium  amalgam, 

C6H6.CH:CH.COOH  +  2H  =  C6H5.CH2.CH2.COOH. 

Synthetically,  it  may  be  obtained  from  the  product  of 
the  action  of  benzyl  chloride  on  the  sodium  compound  of 
ethyl  malonate  (p.  429).  It  crystallises  from  water  in 
needles,  melts  at  47°,  and  distils  at  280°  without  decomposi- 
tion. 

Cinnamic  acid,  or  phenylacrylic  acid,  C6H5-CH;CH.COOH, 
is  closely  related  to  phenylpropionic  acid,  and  is  one  of 
the  best-known  unsaturated  acids  of  the  aromatic  series.  It 
occurs  in  large  quantities  in  storax  (Styrax  officinalis),  and 
may  be  easily  obtained  from  this  resin  by  warming  it  with 
soda ;  the  filtered  aqueous  solution  of  sodium  cinnamate  is 
then  acidified  with  hydrochloric  acid,  and  the  precipitated 
cinnamic  acid  purified  by  recrystallisation  from  boiling 
water. 

Cinnamic  acid  is  usually  prepared  synthetically  by 
heating  benzaldehyde  with  acetic  anhydride  and  anhydrous 


CARBOXYLIC   ACIDS.  431 

sodium  acetate,    a   process   of   condensation   which   is  most 
simply  expressed  by  the  equation, 

C6H5-CHO  +  CH3-COOH  -  C6H5-CH:CH.COOH  +  H20. 

A  mixture  of  benzaldehyde  (3  parts),  acetic  anhydride  (10  parts), 
and  anhydrous  sodium  acetate  (3  parts)  is  heated  to  boiling  in  a 
flask  placed  in  an  oil-bath.  After  about  eight  hours  the  mixture  is 
poured  into  water,  and  distilled  in  steam  to  separate  the  unchanged 
benzaldehyde ;  the  residue  is  then  treated  with  caustic  soda,  the 
hot  alkaline  solution  filtered  from  oily  and  tarry  impurities, 
and  acidified  with  hydrochloric  acid,  the  precipitated 
cinnamic  acid  being  purified  by  recrystallisation  from  boiling 
water. 

This  method  (Perkin's  reaction)  is  a  general  one  for  the  prepara- 
tion of  unsaturated  aromatic  acids,  as  by  employing  the  anhydrides 
and  sodium  salts  of  other  fatty  acids,  homologues  of  cinnamic  acid 
are  obtained.  When,  for  example,  benzaldehyde  is  treated  with 
sodium  propionate  and  propionic  anhydride,  phenylmethylacrylic 
acid  (a.-methylcinnamic  acid),  C6H5-CH:C(CH3)-COOH,  is  formed  ; 
phenylisocrotonic  acid,  C6H5-CH:CH-CH2-COOH,  is  not  obtained 
by  this  reaction,  because  condensation  always  takes  place  between 
the  aldehyde  oxygen  atom  and  the  hydrogen  atoms  of  that 
-CH2-  group,  which  is  directly  united  with  the  carboxyl-radicle. 

Phenylisocrotonic  acid  may,  however,  be  prepared  by  heating 
benzaldehyde  with  a  mixture  of  sodium  succinate  and  succinic 
anhydride, 

C6H5.CHO  +  COOH.CH0.CH,.COOH  = 

C6H5.CH:CH-CH2.COOH  +  CO2  +  H2O. 

It  is  a  colourless,  crystalline  substance,  which  melts  at  86°,  and 
boils  at  302° ;  at  its  boiling-point  it  is  gradually  converted  into 
a-naphthol  and  water  (p.  453). 

Cinnamic  acid  crystallises  from  water  in  needles,  and  melts 
at  133°.  Its  chemical  behaviour  is  in  many  respects  similar 
to  that  of  acrylic  acid  and  other  unsaturated  fatty  acids ;  it 
combines  directly  with  bromine,  for  example,  yielding 
phenyl  aft-dibromopropionic  acid,  C6H5-CHBr-CHBr-COOH, 
and  with  hydrobromic  acid,  giving  plieHyl-fl-bromopropionic 
acid,  C6H5-CHBr.CH2.COOH ;  on  reduction  with  sodium 
amalgam  it  is  converted  into  phenyl  propionic  acid  (p.  430), 
just  as  acrylic  acid  is  transformed  into  propionic  acid. 


432  CARBOXYLIC   ACIDS. 

When  distilled  with  lime,  cinnamic  acid  is  decomposed  into 
carbon  dioxide,  and  phenylethylene  or  styrolene*  just  as 
benzoic  acid  yields  benzene, 

C6H5-CH:CH.COOH  =  C6H5-CH:CH2  +  C02. 

Concentrated  nitric  acid  converts  cinnamic  acid  into  a  mix- 
ture of  about  equal  quantities  of  o-  and  p-nitrocinnamic  acids, 
C6H4(N02).CH:CH-COOH,  which  may  be  separated  by  con- 
version into  their  ethyl  salts,  C6H4(N02).CH:CH-COOC2H5 
(fry  means  of  alcohol  and  hydrogen  chloride),  and  recrystallis- 
ing  these  from  alcohol,  the  sparingly  soluble  ethyl  salt  of  the 
^;-acid  being  readily  separated  from  the  readily  soluble  ethyl 
o-nitrocinnamate.  From  the  pure  ethyl  salts  the  acids  are 
then  regenerated  by  hydrolysing  with  dilute  sulphuric  acid. 
They  resemble  cinnamic  acid  closely  in  properties,  and  combine 
directly  with  bromine,  yielding  the  corresponding  nitrophenyl- 
dibromopropionic  acids,  C6H4(N02)-CHBr.CHBr-COOH. 

Phenylpropiolic  acid,  C6H5-C  :C-COOH,  is  obtained  by  treating 
phenyldibromopropionic  acid,  or,  better,  its  ethyl  salt,  with  alcoholic 
potash, 

C6H5-CHBi-.CHBr.COOH  =  C6H5.C:C-COOH  +  2HBr, 

a  method  which  is  exactly  similar  to  that  employed  in  preparing 
acetylene  by  the  action  of  alcoholic  potash  on  ethylene  dibromide. 
It  melts  at  137°,  and  at  higher  temperatures,  or  when  heated  with 
water  at  120°,  it  decomposes  into  carbon  dioxide  andphenylacetylene, 
a  colourless  liquid,  which  boils  at  140°,  and  is  closely  related  to 
acetylene  in  chemical  properties, 

C6H5-C  IC-COOH  =  C6H5.C!CH  +  CO2. 

o-Nitrophenylpropiolic  acid,  C6H4(NO.2)-C:C-COOH,  maybe  simi- 
larly prepared  from  o-nitroplienylilibromopropiouic  acid;  it  is  a 
substance  of  great  interest,  as  when  treated  with  reducing  agents, 

*  Styrolene,  C6H5-CH:CH2,  may  be  taken  as  a  typical  example  of  an 
aromatic  hydrocarbon  containing  an  unsaturated  side-chain.  It  is  a 
colourless  liquid  which  boils  at  145°,  and  in  chemical  properties  shows  the 
closest  resemblance  to  ethylene,  of  which  it  is  the  phenyl  substitution 
product.  With  bromine,  for  example,  it  yields  a  dibromadditive 
product,  C6H5-CHBr-CH2Br  (dibromethylbenzene),  and  when  heated  with 
hydriodic  acid,  it  is  reduced  to  ethylbenzene, 


CARBOXYLIC    ACIDS.  433 

such  as  hydrogen  sulphide,  or  grape-sugar  and  potash,  it  is  con- 
verted into  indigo  blue  (Baeyer), 

2C6H4<££COOH  +  4H  =  C16H10N202  +  2C02  +  2H2O. 

This  method  of  preparation,   however,   is  not  of  technical  value, 
owing  to  the  high  price  of  phenylpropiolic  acid. 


CHAPTER     XXIX. 

HYDROXYCARBOXYLIC   ACIDS. 

The  hydroxy-acids  of  the  aromatic  series  are  derived  from 
benzole  acid  and  its  homologues,  by  the  substitution  of 
hydroxyl-groups  for  hydrogen  atoms,  just  as  glycollic  acid, 
for  example,  is  derived  from  acetic  acid  (part  i.  p.  225) ;  like 
the  simple  hydroxy-derivatives  of  the  hydrocarbons,  they  may 
be  divided  into  two  classes,  according  as  the  hydroxyl-group  is 
united  with  carbon  of  the  nucleus  or  of  the  side-chain.  In 
the  first  case  the  hydroxyl-group  has  the  same  character 
as  in  phenols,  and  consequently  hydroxy-acids,  of  this  class, 
as,  for  example,  the  three  (o.m.p.)  hydroxyberizoic  acids, 
C6H4(OH)-COOH,  are  both  phenols  and  carboxylic  acids;  in 
the  second  case,  however,  the  hydroxyl-group  has  the  same 
character  as  in  alcohols,  so  that  the  compounds  of  this  class, 
such  as  mandelic  acid,  C6H5-CH(OH)-COOH,  have  properties 
closely  resembling  those  of  the  fatty  hydroxy-acids ;  in  other 
words,  the  differences  between  the  two  classes  of  aromatic 
hydroxy-acids  are  practically  the  same  as  those  between 
phenols  and  alcohols. 

As  those  acids,  which  contain  the  hydroxyl-group  united 
with  carbon  of  the  nucleus,  form  by  far  the  more  important 
class,  the  following  statements  refer  to  them  only,  except 
where  stated  to  the  contrary. 

Preparation. — The  hydroxy-acids  may  be  prepared  from 
the  simple  carboxylic  acids,  by  reactions  exactly  similar  to 
those  employed  in  the  preparation  of  phenols  from  hydro- 

2B 


434  HYDROXYCARBOXYLIC   ACIDS. 

carbons  ;  that  is  to  say,  the  acids  are  converted  into  nitro- 
compounds,  then   into   amido-compounds,   and   the  latter  are 
treated  with  nitrous  acid  in  the  usual  manner, 
r  TT  rorm  _  *  r  TT  XCOOH         r  „  xCOOH 

U6J±5^U<  UgH^-^-Q  °6     4 

/COOH 


or,  the  acids  are  heated  with  sulphuric  acid,  and  the  sulphonic 
acids  obtained  in  this  way  are  fused  with  potash, 

C6H5.COOH — *C61 

It  must  be  borne  in  mind,  however,  that  as  the  carboxyl- 
group  of  the  acid  determines  the  position  taken  up  by  the 
nitro-  and  sulphonic-groups  (p.  352),  only  the  mete-hydroxy- 
compounds  are  conveniently  prepared  in  this  way  directly 
from  the  carboxylic  acids. 

The  or^o-hydroxy-acids,  and  in  some  cases  the  meta-  and 
para-compounds,  are  most  conveniently  prepared  from  the 
phenols  by  one  of  the  following  methods  : 

The  dry  sodium  compound  of  the  phenol  is  heated  at  about 
200°  in  a  stream  of  carbon  dioxide, 


2C6H6.ONa  +  C02  =  C6H4  +  C6H5-OH. 


Under  these  conditions  half  the  phenol  distils  over  and  is  re- 
covered ;  but  if  the  sodium  compound  be  first  saturated  with  carbon 
dioxide  under  pressure,  it  is  converted  into  an  aromatic  deriva- 
tive of  carbonic  acid,  which,  when  heated  at  about  130°  under 
pressure,  is  completely  transformed  into  a  salt  of  the  hydroxy-acid 
by  molecular  change, 

C6H5-ONa  +  CO,  =  C6H5.0-COONa  =  C6H4<^ONa 

Sodium  Phenyl carbonate. 

Many  dihydric  and  trihydric  phenols  may  be  converted 
into  the  corresponding  hydroxy-acids,  simply  by  heating  them 
with  ammonium  carbonate  or  potassium  bicarbonate ;  when 
resorcinol,  for  example,  is  treated  in  this  way,  it  yields  a 
mixture  of  isomeric  resorcylic  acids,  C6H3(OH)2-COOH. 


HYPROXYCARBOXYLIC    ACIDS.  435 

The  second  general  method  of  preparing  hydroxy-acids 
from  phenols  consists  in  boiling  a  strongly  alkaline  solution 
of  the  phenol  with  carbon  tetrachloride  ;  the  principal  product 
is  the  ort?u>-&cid,  but  varying  proportions  of  the  para-acid  are 
also  formed, 

CC14  +  5NaOH  - 


After  the  substances  have  been  heated  together  for  some  hours, 
the  unchanged  carbon  tetrachloride  is  distilled  off,  the  residue 
acidified,  and  the  solution  extracted  with  ether  ;  the  crude  acid, 
obtained  on  evaporating  the  ethereal  solution,  is  then  separated 
from  unchanged  phenol  by  dissolving  it  in  sodium  carbonate,  re- 
precipitated  with  a  mineral  acid,  and  purified  by  recrystallisation. 

The  above  method  is  clearly  analogous  to  Reimer's  reaction 
(p.  409),  and  the  changes  which  occur  during  the  process  may  be 
assumed  to  be  indicated  by  the  following  equations,  in  which  water 
is  represented  instead  of  soda  for  the  sake  of  simplicity  : 


C6H5.OH  +  CC14  .  C6H43  +  HC1 

3HC1 


Properties.  —  The  hydroxy-acids  are  colourless,  crystalline 
substances,  more  readily  soluble  in  water  and  less  volatile 
than  the  acids  from  which  they  are  derived  ;  many  of  them 
undergo  decomposition  on  distillation,  carbon  dioxide  being 
evolved  ;  when  heated  with  lime  they  are  completely  decom- 
posed, with  formation  of  phenols, 

C6H4<OH°H  =  C6H5'OH  +  C02 
C6H3(OH)2.COOH  -  C6H4(OH)2  +  C02. 

The  o-acids,  as,  for  example,  salicylic  acid,  give,  in  neutral 
solution,  a  violet  colouration  with  ferric  chloride,  whereas  the 
m-  and  ^-hydroxy-acids,  such  as  the  ra-  and  ^-hydroxybenzoic 
acids,  give  no  colouration, 


436  HYDROXYCABBOXYLIC    ACIDS. 

The  chemical  properties  of  the  hydroxy-acids  will  be  readily 
understood,  when  it  is  remembered  that  they  are  both  phenols 
and  carboxylic  acids.  As  carboxylic  acids  they  form  salts  by 
the  displacement  of  the  hydrogen  atom  of  the  carboxyl-group, 
such  salts  being  obtained  on  treating  with  carbonates  or  with 
the  calculated  quantity  of  the  metallic  hydroxide  ;  when, 
however,  excess  of  alkali  hydroxide  is  employed,  the  hydrogen 
of  the  hydroxyl-group  is  also  displaced,  just  as  in  phenols. 
It  is  clear,  therefore,  that  hydroxy-acids  form  both  mono-  and 
di-metallic  salts,  salicylic  acid,  for  example,  yielding  the  two 
sodium  salts,  C6H4(OH)-COONa  and  C6H4(ONa)-COONa. 

The  di-metallic  salts  are  decomposed  by  carbon  dioxide, 
with  formation  of  mono-metallic  salts,  just  as  the  phenates 
are  resolved  into  the  phenols  ;  the  metal  in  combination  with 
the  carboxyl-group,  however,  cannot  be  displaced  in  this  way. 

The  ethereal  salts  of  the  hydroxy-acids  are  prepared  in  the 
usual  manner  —  namely,  by  saturating  a  solution  of  the  acid  in 
the  alcohol  with  hydrogen  chloride  (part  i.  p.  187)  ;  by  this 
treatment  the  hydrogen  of  the  carboxyl-group  only  is 
displaced,  normal  ethereal  salts,  such  as  methyl  salicylate, 
C6H4(OH)-COOCH3,  being  formed;  these  compounds  have 
still  phenolic  properties,  and  dissolve  in  caustic  alkalies,  form- 
ing metallic  derivatives,  such  as  methyl  potassiosalicylate, 
C6H4(OK)-COOCH3,  which,  when  heated  with  alkyl  halogen 
compounds,  yield  alkyl-derivatives,  such  as  methyl  metliyl- 
salicylate,  C6H4(OCH3).COOCH3.  On  hydrolysing  di-alkyl 
compounds  of  this  kind  with  alcoholic  potash,  only  the  alkyl 
of  the  carboxyl-group  is  removed,  methyl  methylsalicylate,  for 
example,  yielding  the  potassium  salt  of  methylsalicylic  acid, 


+  KOH  =  C 

The  other  alkyl-group  is  not  eliminated  even  on  boiling 
with  alkalies,  a  behaviour  which  corresponds  with  that  of 
the  alkyl-group  in  derivatives  of  phenols,  such  as  anisole, 
C6H5-OCH3  (p.  392)  ;  just,  however,  as  anisole  is  decomposed 


HYDROXYCARBOXYLIC    ACIDS.  437 

into  phenol  and  methyl  iodide  when  heated  with  hydriodic 
acid,  so  methylsalicylic  acid  under  similar  conditions  yields 
the  hydroxy-acid, 


p 

.  6 

3 

Salicylic  acid,  or  o-hydroxybenzoic  acid,  C6H4(OH)-COOH, 
occurs  in  the  blossom  of  Spiraea  ulmaria,  and  is  also  found 
in  considerable  quantities,  as  methyl  salicylate,  in  oil  of 
wintergreen  (Gaultheria  procumbens).  It  used  to  be  pre- 
pared, especially  for  pharmaceutical  purposes,  by  hydrolysing 
this  oil  with  potash  ;  after  boiling  off  the  methyl  alcohol 
(part  i.  p.  88),  the  solution  is  acidified  with  dilute  sulphuric 
acid,  and  the  precipitated  salicylic  acid  purified  by  recrystal- 
lisation  from  water. 

Salicylic  acid  may  be  obtained  by  oxidising  salicylalde- 
hyde  (p.  409)  or  salicylic  alcohol  (saligenin,  p.  404)  with 
chromic  acid,  by  treating  o-amidobenzoic  acid  (anthranilic 
acid)  with  nitrous  acid,  and  also  by  boiling  phenol  with  soda 
and  carbon  tetrachloride. 

It  is  now  prepared  on  the  large  scale  by  treating  sodium 
phenate  with  carbon  dioxide  under  pressure,  and  then  heating 
the  sodium  phenylcarbonate,  C6H5-0-COONa,  which  is  thus 
formed,  at  120-140°  under  pressure,  when  it  undergoes  intra- 
molecular change  into  sodium  salicylate  (p.  434). 

Salicylic  acid  is  sparingly  soluble  in  cold  (1  in  400  parts  at 
15°),  but  readily  in  hot,  water,  from  which  it  crystallises  in 
needles,  melting  at  156°;  its  neutral  solutions  give  with  ferric 
chloride  an  intense  violet  colouration.  When  rapidly  heated 
it  sublimes,  and  only  slight  decomposition  occurs  ;  but  when 
distilled  slowly,  a  large  proportion  decomposes  into  phenol 
and  carbon  dioxide,  this  change  being  complete  if  the  acid  be 
distilled  with  lime.  All  these  properties  serve  for  the  detec- 
tion of  salicylic  acid. 

Salicylic  acid  is  a  powerful  antiseptic,  and,  as  it  has  no 
smell,  it  is  frequently  used  as  a  disinfectant  instead  of 


438  HYDROXYCARBOXYLIC    ACIDS. 

phenol  ;  it  is  also  extensively  employed  in  medicine  and  as  a 
food  preservative.  The  mono-metallic  salts  of  salicylic  acid, 
as,  for  example,  potassium  salicylate,  C6H4(OH)-COOK,  and 
calcium  salicylate,  {C6H4(OH)-COO}2Ca,  are  prepared  by 
neutralising  a  hot  aqueous  solution  of  the  acid  with  metallic 
carbonates;  they  are,  as  a  rule,  soluble  in  water.  The  di- 

metallic  salts,  such  as  C6H4(OK).COOK  and  C6H4<^^>Ba, 

are  obtained  in  a  similar  manner,  employing  excess  of  the 
metallic  hydroxides;  with  the  exception  of  the  salts  of  the 
alkali  metals,  these  di-metallic  compounds  are  insoluble  ;  they 
are  all  decomposed  by  carbon  dioxide,  with  formation  of  the 
mono-metallic  salts, 


Methyl  salicylate,  C6H4(OH).COOCH3,  prepared  in  the 
manner  described  (p.  436),  or  by  distilling  a  mixture  of  salicylic 
acid,  methyl  alcohol,  and  sulphuric  acid  (part  i.  p.  188),  is  an 
agreeably-smelling  oil,  boiling  at  224°  ;  ethyl  salicylate, 
C6H4(OH)-COOC2H5,  boils  at  223°. 

Methyl  methylsalicylate,  C6H4(OCH3)-COOCH3,  is  formed  when 
methyl  salicylate  is  heated  with  methyl  iodide  and  alcoholic  potash 
(1  niol.)  ;  it  is  an  oil  boiling  at  228°. 

Methylsalicylic  acid,  C6H4(OCH3)-COOH,  is  obtained  when  its 
methyl  salt  is  hydrolysed  with  potash  ;  it  is  a  crystalline  substance, 
melting  at  98-5°,  and  when  heated  with  hydriodic  acid  it  is  de- 
composed, giving  salicylic  acid  and  methyl  iodide;  the  other 
halogen  acids  have  a  similar  action. 

m-Hydroxybenzoic  acid  is  prepared  by  fusing  m-sulphoberrzoic 
acid  with  potash,  and  also  by  the  action  of  nitrous  acid  on 
w-amidobenzoic  acid  (p.  422).  It  melts  at  200°,  gives  no  coloura- 
tion with  ferric  chloride,  and  when  distilled  with  lime  it  is  decom- 
posed into  phenol  and  carbon  dioxide. 

p-Hydroxybenzoic  acid  is  formed,  together  with  salicylic  acid,  by 
the  action  of  carbon  tetrachloride  and  soda  on  phenol  ;  it  may  also 
be  obtained  from  ;?-sulphobenzoic  acid  by  fusion  with  potash,  or  by 
the  action  of  nitrous  acid  on  jt?-amidoberizoic  acid. 

It  is  prepared  by  heating  potassium  phenate  in  a  stream  of  carbon 


HYDROXYCARBOXYLIC   ACIDS.  439 

dioxide  at  220°  as  long  as  phenol  distils  over;  if,  however,  the 
temperature  be  kept  below  150°,  potassium  salicylate  is  formed  ;  the 
residue  is  dissolved  in  water,  the  acid  precipitated  from  the 
filtered  solution  by  adding  hydrochloric  acid,  and  purified  by  re- 
crystallisation  from  water.  ^-Hydroxybenzoic  acid  melts  at  210°, 
and  is  completely  decomposed  on  distillation  into  phenol  and 
carbon  dioxide ;  its  aqueous  solution  gives  no  colouration  with 
ferric  chloride. 

Anisic  acid, £>-methoxybenzoic  acid,  C6H4(OCH3)-COOH,is 
obtained  by  oxidising  anethole,  C6H4(OCH3).CH:CH-CH3  (the 
principal  constituent  of  oil  of  aniseed)  with  chromic  acid,  when 
the  group  -CH:CH-CH3  is  converted  into  -COOH  (p.  410) ;  it 
may  also  be  prepared  from  j9-hydroxybenzoic  acid  by  a  series 
of  reactions  analogous  to  those  employed  in  the  formation  of 
methylsalicylic  acid  from  salicylic  acid  (see  above).  Anisic 
acid  melts  at  185°,  and  when  distilled  with  lime  it  is  decom- 
posed, with  formation  of  anisole  (p.  392) ;  when  heated  with 
fuming  hydriodic  acid,  it  yields  £>-hydroxybenzoic  acid  and 
methyl  iodide. 

There  are  six  dihydroxylenzvic  acids,  C6H3(OH)2-COOH, 
two  of  which  are  derived  from  catechol,  three  from  resorcinol, 
and  one  from  hydroquinone  ;  the  most  important  of  these  is 
protocatechuic  acid,  [OH:OH:COOH  -  1:2:4],  one  of  the 
two  isomeric  catecholcarboxylic  acids.  This  compound  is 
formed  on  fusing  many  resins,  such  as  catechu  and  gum 
benzoin,  and  also  certain  alkaloids,  with  potash,  and  it  may 
be  prepared  synthetically  by  heating  catechol  with  water  and 
ammonium  carbonate  r.t  140°. 

It  crystallises  from  water,  in  which  it  is  very  soluble,  in 
needles,  melts  at  199°,  and  when  strongly  heated  it  is  decom- 
posed into  catechol  and  carbon  dioxide ;  its  aqueous  solution 
gives  with  ferric  chloride  a  green  solution,  which  becomes 
violet  and  then  red  on  the  addition  of  sodium  bicarbonate. 

Gallic  acid,  or  pyrogallolcarboxylic  acid, 

C6H2(OH)3-COOH  [OH:OH:OH:COOH  =  1:2:3:5], 
is  a  trihydroxybenzoic  acid ;  it  occurs  in  gall-nuts,  tea,  and 


440  HYDROXYCARBOXYLIC    ACIDS. 

many  other  vegetable  products,  and  is  best  prepared  by  boiling 
tannin  (see  below)  with  dilute  acids.  It  crystallises  in 
needles,  and  melts  at  220°,  being  at  the  same  time  resolved  into 
pyrogallol  (p.  400)  and  carbon  dioxide  ;  it  is  readily  soluble 
in  water,  and  its  aqueous  solution  gives  with  ferric  chloride  a 
bluish-black  precipitate.  Gallic  acid  is  a  strong  reducing 
agent,  and  precipitates  gold,  silver,  and  platinum  from  solutions 
of  their  salts. 

Tannin,  digallic  acid,  or  tannic  acid,  C14H1009,  occurs  in 
large  quantities  in  gall-nuts,  and  in  all  kinds  of  bark,  from 
which  it  may  be  extracted  with  boiling  water.  It  is  an 
almost  colourless,  amorphous  substance,  and  is  readily  soluble 
in  water ;  its  solution  possesses  a  very  astringent  taste,  and 
gives  with  ferric  chloride  an  intense  dark-blue  solution,  for 
which  reason  tannin  is  largely  used  in  the  manufacture  of 
inks. 

When  boiled  with  dilute  sulphuric  acid,  tannin  is  completely 
converted  into  gallic  acid,  a  fact  which  shows  that  it  is  the 
anhydride  of  this  acid, 

C14H1009  +  H20  =  2C7H605. 

Tannin  is  used  largely  in  dyeing  as  a  mordant,  owing  to  its 
property  of  forming  insoluble  coloured  compounds  with  many 
dyes.  It  is  also  extensively  employed  in  '  tanning ;'  when 
animal  skin  or  membrane  is  placed  in  a  solution  of  tannin,  or 
in  contact  with  moist  bark  containing  tannin,  it  absorbs 
and  combines  with  the  tannin,  and  is  converted  into  a 
much  tougher  material;  such  tanned  skins  constitute 
leather. 

Mandelic  acid,  C6H5.CH(OH)-COOH  (phenylglycollic  acid), 
is  an  example  of  an  aromatic  hydroxy-acid  containing  the 
hydroxyl-group  in  the  side-chain.  It  may  be  obtained  by 
boiling  amygdalin  (which  yields  benzaldehyde,  hydrogen 
cyanide,  and  glucose,  p.  405)  with  hydrochloric  acid,  but  it 
is  usually  prepared  by  treating  benzaldehyde  with  hydrocyanic 
acid  and  hydrolysing  the  resulting  hydroxycyanide,  a  method 


HYDROXYCARBOXYLIC    ACIDS.  441 

analogous  to  that  employed  in  the  synthesis  of  lactic  acid  from 
aldehyde  (part  i.  p.  139), 


C6H5-CHO  +  HCN  =  C6 
C6H5.CH(OH).CN  +  2H20  =  C6H5-CH(OH).COOH  +  NH3. 

Mandelic  acid  melts  at  133°,  is  moderately  soluble  in  water, 
and  shows  in  many  respects  the  greatest  resemblance  to  lactic 
acid  (methylglycollic  acid)  ;  when  heated  with  hydriodic 
acid,  for  example,  it  is  reduced  to  phenylacetic  acid  (p.  429), 
just  as  lactic  acid  is  reduced  to  propionic  acid  (part  i.  p.  227), 

C6H5.CH(OH).COOH  +  2HI  -  C6H5.CH2-COOH  +  I2  +  H2O. 

The  character  of  the  hydroxyl-group  in  mandelic  acid  is,  in 
fact,  quite  similar  to  that  of  the  hydroxyl-group  in  the  fatty 
hydroxy-acids  and  in  the  alcohols,  so  that  there  are  many 
points  of  difference  between  mandelic  acid  and  acids,  such  as 
salicylic  acid,  which  contain  the  hydroxyl-group  united  with 
carbon  of  the  nucleus  ;  when,  for  example,  ethyl  mandelate, 
C6H5-CH(OH).COOC2H5,  is  treated  with  caustic  alkalies,  it 
does  not  yield  an  alkali  derivative,  although  the  hydrogen  of 
the  hydroxyl-group  is  displaced  on  treating  with  sodium  or 
potassium. 

Mandelic  acid,  like  lactic  acid,  contains  an  asymmetric 
carbon  atom  (p.  533),  and  can,  therefore,  exist  in  three 
optically  different  forms.  The  synthetical  acid  is  optically 
inactive  —  that  is  to  say,  it  is  a  mixture  of  the  dextro-  and  levo- 
rotatory  acids,  but  the  acid  prepared  from  amygdalin  is  levo- 
rotatory.  The  dextro-rotatory  acid  may  be  obtained  by 
growing  the  organism  Penicttiium  glaucum  in  a  solution  of 
the  inactive  acid  under  suitable  conditions,  when  the  levo- 
rotatory  acid  is  destroyed,  the  dextro-rotatory  acid  remaining 
(p.  544). 


442  NAPHTHALENE  AND  ITS  DERIVATIVES. 

CHAPTER    XXX. 

NAPHTHALENE  AND  ITS  DERIVATIVES. 

All  the  aromatic  hydrocarbons  hitherto  described,  with  the 
exception  of  diphenyl,  diphenylmethane,  and  triphenylmethane 
(p.  340),  contain  only  one  closed-chain  of  six  carbon  atoms,  and 
are  very  closely  and  directly  related  to  benzene ;  most  of  them 
may  be  prepared  from  benzene  by  comparatively  simple 
reactions,  and  reconverted  into  this  hydrocarbon,  perhaps 
even  more  readily,  so  that  they  may  all  be  classed  as  simple 
benzene  derivatives.  The  exceptions  just  mentioned  are 
also,  strictly  speaking,  derivatives  of  benzene,  although  at  the 
same  time  they  may  be  regarded  as  hydrocarbons  of  quite 
another  class,  since  diphenyl  and  diphenylmethane  contain 
two,  and  triphenylmethane  three,  closed-chains  of  six  carbon 
atoms.  There  are,  in  fact,  numerous  classes  or  types  of 
aromatic  hydrocarbons,  and,  just  as  benzene  is  the  first 
member  of  a  homologous  series  and  the  parent  substance  of 
a  vast  number  of  derivatives,  so  also  these  other  hydrocarbons 
form  the  starting-points  of  new  homologous  series  and  of 
derivatives  of  a  different  type. 

The  hydrocarbons  naphthalene  and  anthracene,  which  are 
now  to  be  described,  are  perhaps  second  only  to  benzene  in 
importance ;  each  forms  the  starting-point  of  a  great  number 
of  compounds,  many  of  which  are  extensively  employed  in  the 
manufacture  of  dyes. 

Naphthalene,  C10H8,  occurs  in  coal-tar  in  larger  quantities 
than  any  other  hydrocarbon,  and  is  easily  isolated  from  this 
source  in  a  pure  condition ;  the  crystals  of  crude  naphthalene, 
which  are  deposited  on  cooling  from  the  fraction  of  coal-tar 
passing  over  between  170  and  230°  (p.  297),  are  first  pressed  to 
get  rid  of  liquid  impurities,  and  then  warmed  with  a  small 
quantity  of  concentrated  sulphuric  acid,  which  converts  most 
of  the  foreign  substances  into  non-volatile  sulphonic  acids; 


NAPHTHALENE    AND    ITS    DERIVATIVES.  443 

the  naphthalene  is  then  distilled  in  steam,  or  sublimed,  and 
is  thus  obtained  almost  chemically  pure. 

Naphthalene  crystallises  in  large,  lustrous  plates,  melts  at 
80°,  and  boils  at  218°.  It  has  a  highly  characteristic  smell, 
and  is  extraordinarily  volatile,  considering  its  high  molecular 
weight,  so  much  so,  in  fact,  that  only  part  of  the  naphtha- 
lene in  crude  coal-gas  is  deposited  in  the  condensers  (p.  295), 
the  rest  being  carried  forward  into  the  purifiers,  and  even 
into  the  gas-mains,  in  which  it  is  deposited  in  crystals  in 
cold  weather,  principally  at  the  bends  of  the  pipes,  frequently 
causing  stoppages.  It  is  insoluble  in  water,  but  dissolves 
freely  in  hot  alcohol  and  ether,  from  either  of  which  it 
may  be  crystallised.  Like  many  other  aromatic  hydrocarbons, 
it  combines  with  picric  acid,  when  the  two  substances  are 
dissolved  together  in  alcohol,  forming  naphthalene  picrate,  a 
yellow  crystalline  compound  of  the  composition, 

C10H8,C6H2(N02)3.OH, 

which  melts  at  149°. 

As  the  vapour  of  naphthalene  burns  with  a  highly  luminous 
flame,  the  hydrocarbon  is  used  to  some  extent  for  carburetting 
coal-gas — that  is  to  say,  for  increasing  its  illuminating  power ; 
for  this  purpose  the  gas  is  passed  through  a  vessel  which 
contains  coarsely-powdered  naphthalene,  gently  heated  by 
the  gas  flame,  so  that  the  hydrocarbon  volatilises  and 
burns  with  the  gas.  The  principal  use  of  naphthalene, 
however,  is  for  the  manufacture  of  a  number  of  derivatives 
which  are  employed  in  the  colour  industry. 

Constitution. — Naphthalene  has  the  characteristic  properties 
of  an  aromatic  compound — that  is  to  say,  its  behaviour  under 
various  conditions  is  similar  to  that  of  benzene  and  its 
derivatives,  and  different  from  that  of  fatty  compounds ; 
when  treated  with  nitric  acid,  for  example,  it  yields  a 
nitro-derivative,  and  with  sulphuric  acid  it  gives  sulphonic 
acids.  This  similarity  between  benzene  and  naphthalene  at 
once  suggests  a  resemblance  in  constitution,  a  view  which  is 


444  NAPHTHALENE    AND    ITS    DERIVATIVES. 

confirmed  by  the  fact  that  naphthalene,  like  benzene,  is  a  very 
stable  substance,  and  is  resolved  into  simpler  substances  only 
with  difficulty.  When,  however,  naphthalene  is  boiled  with 
dilute  nitric  or  chromic  acid,  it  is  slowly  oxidised,  yielding 
carbon  dioxide  and  (or^o)-phthalic  acid,  C6H4(COOH)2. 

Now  the  formation  of  phthalic  acid  in  this  way  is  a  fact  of 
very  great  importance,  since  it  is  a  proof  that  naphthalene 
contains  the  group, 


C 

that  is  to  say,  that  it  contains  a  benzene  nucleus  to  which  two 
carbon  atoms  are  united  in  the  ortho-position  to  one  another. 
Nevertheless,  further  evidence  is  required  in  order  to  arrive  at 
the  constitution  of  the  hydrocarbon,  since  there  are  still  two 
carbon  and  four  hydrogen  atoms  to  be  accounted  for,  and 
there  are  many  different  ways  in  which  these  might  be  united 


with  the  C6H4XQ  group. 

Clearly,  therefore,  it  is  important  to  ascertain  the  structure 
of  that  part  of  the  naphthalene  molecule  which  has  been  oxi- 
dised to  carbon  dioxide  and  water  —  to  obtain,  if  possible,  some 
simple  decomposition  product  in  which  these  carbon  and  hydro- 
gen atoms  are  retained  in  their  original  state  of  combination. 

Now  this  can  be  done  in  the  following  way  :  When 
nitronaphthalene,  C10H7-N02,  a  simple  mono-substitution 
product  of  the  hydrocarbon,  is  boiled  with  dilute  nitric  acid, 
it  yields  nitrophthalic  acid,  C6H3(N02)(COOH)2  ;  therefore, 
again,  naphthalene  contains  a  benzene  nucleus,  and  the  nitro- 
group  in  nitronaphthalene  is  combined  with  this  nucleus. 
If,  however,  the  same  nitronaphthalene  be  reduced  to  amido- 
naphthalene,  C10H7-NH2,  and  the  latter  oxidised,  phthalic 
acid  (and  not  amidophthalic  acid)  is  obtained  ;  this  fact  can 
only  be  explained  by  assuming  either  that  the  benzene  nucleus, 
which  is  known  to  be  united  with  the  amido-group,  has  been 


NAPHTHALENE    AND    ITS    DERIVATIVES. 


445 


destroyed,  or  that  the  amido-group  has  been  displaced  by 
hydrogen  during  oxidation.  Since,  however,  the  latter 
alternative  is  contrary  to  all  experience,  the  former  must  be 
accepted,  and  it  is  clear  that  the  benzene  nucleus  which  is 
contained  in  the  oxidation  product  of  amidonaphthalene  is 
not  the  same  as  that  present  in  the  oxidation  product  of 
nitronaphthalene ;  in  other  words,  different  parts  of  the 
naphthalene  molecule  have  been  oxidised  to  carbon  dioxide 
and  water  in  the  two  cases,  and  yet  in  both  the  group 


remains.     The    constitution   of  naphthalene   must 

therefore  be  expressed  by  the  formula 
CH          CH 
JX 


CH 


This  will  be  evident  if  the  above  changes  be  expressed 
with  the  aid  of  this  formula.  When  nitronaphthalene  is 
oxidised,  the  nucleus  B  (see  below),  which  does  not  contain 
the  nitro-group,  is  destroyed,  as  indicated  by  the  dotted 
lines,  the  product  being  nitrophthalic  acid;  when,  on  the 
other  hand,  amidonaphthalene  is  oxidised,  the  nucleus  A, 
combined  with  the  amido-group,  is  attacked  and  destroyed 
in  preference  to  the  other,  and  phthalic  acid  is  formed, 

N02  N02 

COOH 


Naphthalene. 

NH2 


Nitronaphthalene. 
COOH 


COOH 
Nitrophthalic  Acid. 


COOH" 

Plithalic  Acid. 


Amidonaphthalene. 

The  constitution  of  naphthalene  was  first  established  in  this 


446  NAPHTHALENE    AND    ITS    DERIVATIVES. 

way  by  Graebe  in  1880,  although  the  above  formula  had 
been  suggested  by  Erlenmeyer  as  early  as  1866;  that  the 
hydrocarbon  is  composed  of  two  benzene  nuclei  partially  super- 
posed or  condensed  together  in  the  o-position,  as  shown  above, 
has  since  been  confirmed  by  syntheses  of  its  derivatives,  but 
even  more  conclusively  by  the  study  of  the  isomerism  of  its 
substitution  products. 

The  difficulty  of  determining  and  of  expressing  the  actual  state  or 
disposition  of  the  fourth  affinity  of  each  of  the  carbon  atoms  in 
naphthalene  is  just  as  great  as  in  the  case  of  benzene.  If  the 
carbon  atoms  be  represented  as  united  by  alternate  double  Unkings, 
as  in  the  formula  on  the  left-hand  side  (see  below),  there  is  the 
objection  that  they  do  not  show,  as  indicated,  the  behaviour  of 
carbon  atoms  in  fatty  unsatu rated  compounds,  as  explained  more 
fully  in  the  case  of  benzene.  For  this  reason  the  formula  on  the 
right-hand  side  (see  below)  has  been  suggested  as  perhaps  prefer- 
able, the  lines  drawn  towards  the  centres  of  the  nuclei  having  the 
same  significance  as  in  the  centric  formula  for  benzene  (p.  307). 
The  simple,  double-hexagon  formula  given  above  is  usually  em- 
ployed for  the  sake  of  convenience. 


Naphthalene  may  be  obtained  synthetically  by  passing  the 
vapour  of  phenylbutylene,  C6H5.CH2.CH2-CH:CH2*  (or  of 
pheriylbutylene  dibromide,  C6H5-CH2.CH2.CfeBr.CH2Br), 
over  red-hot  lime,  the  change  being  a  process  of  destructive 
distillation,  accompanied  by  loss  of  hydrogen,  similar  to,  but 
much  simpler  than  that  which  occurs  in  the  formation  of 
other  aromatic  from  fatty  hydrocarbons  (p.  300), 

/CH:CH 

C6H5.CH2.CH2.CH:CH2  -  C6H4<f         |       +  2H2. 

CH:CH 

*  Phenylbutylene  is  obtained  by  treating  a  mixture  of  benzyl  chloride 
and  allyl  iodide  with  sodium, 

C6H5  CH2C1  +  CH2I-CH:CH2  +  2Na  =  C6H5.CH2-CH2-CH:CH2  +  NaCl  +  Nal. 
It  is  a  liquid,  boiling  at  178°,  and,  like  butylene  (part  i.  p.  79),  it  combines 
directly  with  one  molecule  of  bromine,  yielding  the  dibromide. 


NAPHTHALENE    AND    ITS    DEKIVATIVES. 


447 


A  most  important  synthesis  of  naphthalene  was  accom- 
plished 1>y  Fittig,  who  showed  that  a-naphthol  (a-hydroxy- 
naphthalene)  is  formed  on  boiling  phenylsscrotonic  acid 
(p.  431)  with  water.  This  change  probably  takes  place  in  two 
stages,  the  first  product  being  a  keto-derivative  of  naphtha- 
lene, which  passes  into  a-naphthol  by  intramolecular  change 
(compare  part  i.  p.  195), 

CH 


CH 


C(OH) 


The  a-naphthol  thus  obtained  is  converted  into  naphtha- 
lene on  distillation  with  zinc-dust,  just  as  phenol  is  trans- 
formed into  benzene  (p.  331). 

Isomerism  of  Naphthalene  Derivatives. — As  in  the  case  of 
benzene,  the  study  of  the  isomerism  of  the  substitution  pro- 
ducts of  naphthalene  affords  the  most  convincing  evidence 
that  the  accepted  constitutional  formula  is  correct.  In  the 
first  place,  naphthalene  differs  from  benzene  in  yielding  two 
different  series  of  mono-substitution  products ;  there  are,  for 
example,  two  monochloronaphthalenes,  two  monohydroxy- 
naphthalenes,  two  mononitronaphthalenes,  &c.  This  fact 
is  readily  accounted  for,  as,  on  considering  the  constitutional 
formula  of  naphthalene,  which  may  be  conveniently  written 


or 


numbered  or  lettered  as  shown  (the  symbols  C  and  H  being 
omitted  for  the  sake  of  simplicity),  it  will  be  evident  that  the 
eight  hydrogen  atoms  are  not  all  similarly  situated  relatively 


448  NAPHTHALENE    AND    ITS    DERIVATIVES. 

to  the  rest  of  the  molecule.  If,  for  example,  the  hydrogen 
atom  (1)  were  displaced  by  chlorine,  hydroxyl,  &c.,  the  substi- 
tution product  would  be  isomeric,  but  not  identical  with  that 
produced  by  the  displacement  of  the  hydrogen  atom  (2).  In 
the  first  case,  the  substituting  atom  or  group  would  be  united 
with  a  carbon  atom  which  is  itself  directly  united  with  a 
carbon  atom  common  to  both  nuclei,  whereas  in  the  other  case 
this  would  not  be  so.  Clearly,  then,  the  fact  that  the  mono- 
substitution  products  of  naphthalene  exist  in  two  isomeric 
forms  is  in  accordance  with  the  above  constitutional  formula. 
Further,  it  will  be  seen  that  not  more  than  two  such  isomerides 
could  be  obtained,  because  the  positions  1.4.1 '.4'  (the  four 
a-positions)  are  identical,  and  so  also  are  the  positions  2.3.2'.3' 
(the  four  /^-positions) ;  the  isomeric  mono-substitution  products 
are,  therefore,  usually  distinguished  by  using  the  letters  a 
and  ft. 

When  two  hydrogen  atoms  in  naphthalene  are  displaced  by 
two  identical  groups  or  atoms,  ten  isomeric  di-derivatives  may 
be  obtained.  Denoting  the  positions  of  the  substitueiits  by  the 
system  of  numbering  shown  above,  these  isomerides  would  be 

1:2,  1:3,  1:4,  1:4',  1:3',  1:2',  1:1',  2:3,  2:3',  2:2', 
all  other  possible  positions  being  identical  with  one  of  these  ; 
2:4',  for  example,  is  the  same  as  1:3',  2': 4  and  3:1',  and  l':4  is 
identical  with  1 : 4'.  The  constitution  of  such  a  di-derivative 
is  usually  expressed  with  the  aid  of  numbers,  as  it  is  necessary 
to  show  whether  the  substituents  are  combined  with  the 
same,  or  with  different,  nuclei. 

When  the  two  atoms  or  groups  are  present  in  the  same 
nucleus,  their  relative  position  is  similar  to  the  0-,  m-,  or 
^-position  in  benzene.  The  positions  1:2,  2:3,  and  3:4  corre- 
spond with  the  ortho-,  1 : 3,  and  2 : 4,  with  the  meta-,  and  1 : 4 
with  the  para-position,  and  similarly  in  the  case  of  the  other 
nucleus.  The  position  1:1'  or  4:4',  however,  is  different  from 
any  of  these,  and  is  termed  the  peri-position ;  groups  thus 
situated  behave  in  much  the  same  way  as  those  in  the 
o-position  in  the  benzene  and  naphthalene  nuclei. 


NAPHTHALENE   AND    ITS    DERIVATIVES.  449 

Derivatives  of  Naphthalene. 

The  homologues  of  naphthalene — that  is  to  say,  its  alkyl 
substitution  products,  are  of  comparatively  little  importance, 
but  it  may  be  mentioned  that  they  may  be  prepared  from  the 
parent  hydrocarbon  by  methods  similar  to  those  employed  in 
the  case  of  the  corresponding  benzene  derivatives,  as,  for 
example,  by  treating  naphthalene  with  alkyl  halogen  com- 
pounds and  aluminium  chloride, 

C10H8  +  C2H5I  =  C10H..C2H5  +  HI, 

and  by  treating  the  bromonaphthalenes  with  an  alkyl  halogen 
compound  and  sodium, 

C10H7Br  +  CH3Br  +  2Na  =  C10HrCHs  +  2NaBr. 

a-Methylnaphthalene,  C10Hr-CH3,  is  a  colourless  liquid, 
boiling  at  240-242°,  but  /3-methylnaphthalene  is  a  solid, 
melts  at  32°,  and  boils  at  242° ;  both  these  hydrocarbons 
occur  in  coal-tar. 

The  halogen  mono -substitution  products  of  naphthalene  are 
also  of  little  importance.  They  may  be  obtained  by  treating 
the  hydrocarbon,  at  its  boiling-point,  with  the  halogens  (chlor- 
ine and  bromine),  but  only  the  a-derivatives  are  formed  in  this 
way.  Both  the  a-  and  the  /^-compounds  may  be  obtained  by 
treating  the  corresponding  naphthols  (p.  452),  or,  better, 
the  naphthalenesulphonic  acids  (p.  455)  with  pentachloride 
or  pentabromide  of  phosphorus, 

C10H7.S02C1  +  PC15  -  C10H7C1  +  POC13  +  SOC12, 

or  by  converting  the  naphthylamines  (p.  452)  into  the  corre- 
sponding diazo-compounds,  and  decomposing  the  latter  with  a 
halogen  cuprous  salt  (p.  372), 

C10H7-NH2  — >  C10Hr.N:NCl  — >  C10H7C1. 

All  these  methods  correspond  with  those  described  in  the 
case  of  the  halogen  derivatives  of  benzene,  and  are  carried  out 
practically  in  a  similar  manner. 

a-Chloronaphthalene,  C10H-C1,  is  a  liquid,  boiling  at  about 
'2  C 


450  NAPHTHALENE   AND    ITS    DERIVATIVES. 

263°,  but  the  ^-derivative  is  a  crystalline  substance,  melting  at 
56°,  and  boiling  at  264°. 

a-Bromonaphthalene,  C10H^Br,  is  also  a  liquid,  which  boils 
at  280°,  but  the  ^-derivative  is  crystalline,  and  melts  at  68°. 

The  chemical  properties  of  these,  and  of  other  halogen 
derivatives  of  naphthalene,  are  similar  to  those  of  the  halogen 
derivatives  of  benzene  ;  the  halogen  atoms  are  very  firmly 
combined,  and  are  not  displaced  by  hydroxyl-groups  on 
boiling  with  alkalies,  &c. 

Naphthalene  tetrachloride,  C10H8C14,  is  an  important  halo- 
gen additive  product,  which  is  produced  on  passing  chlorine 
into  a  vessel  containing  coarsely-powdered  naphthalene  at 
ordinary  temperatures.  It  forms  large  colourless  crystals, 
melts  at  182°,  and  is  converted  into  dichloronaphtlialene 
C10H6C12  (a  substitution  product),  when  heated  with  alco- 
holic potash  ;  it  is  readily  oxidised  by  nitric  acid,  yielding 
phthalic  and  oxalic  acids,  a  fact  which  shows  that  all  the 
chlorine  atoms  are  present  in  one  and  the  same  nucleus  ;  the 
constitution  of  the  compound  is  therefore  expressed  by  the 

t        i    P  TI  x-CHCl-CHCl>. 
formula  C6H4<CHcl.CHCi> 

The  formation  of  this  additive  product  shows  tli.it  naphthalene, 
like  benzene,  is  not  really  a  saturated  compound,  although  it  usually 
behaves  as  such  ;  other  compounds,  formed  by  the  addition  of  four 
atoms  of  hydrogen  to  naphthalene  or  to  a  naphthalene  derivative, 
are  known,  and  experience  has  shown  that  when  one  of  the  nuclei 
is  thus  fully  reduced,  the  atoms  or  groups  of  which  it  is  composed 
acquire  the  character  which  they  have  in  fatty  compounds,  whereas 
the  unreduced  nucleus  retains  the  character  of  that  in  benzene. 
The  amido-group  in  the  tetrahydro-fi-naphthylamine  of  the  consti- 


/s-., 
tution  CfiH4<  ",  for  example,  has  the  same  character  as 

XCH2-CH2 
that  in  fatty  amines,  whereas  in  the  case  of  the  isomeric  tetrahydro- 

/CH2.CH2 
fi-naphthylamine,    NH2-C6H3\       "  |      ,  the   amido-group  has  the 

CH2-CH2 

same  properties  as  that  in  aniline,  because  it  is  combined  with 
the  unreduced  nucleus. 


NAPHTHALENE    AND    ITS    DERIVATIVES.  451 

Nitro- derivatives. — Naphthalene,  like  benzene,  is  readily 
acted  on  by  concentrated  nitric  acid,  yielding  nitro-derivatives, 
one,  two,  or  more  atoms  of  hydrogen  being  displaced  accord- 
ing to  the  concentration  of  the  acid  employed  and  the 
temperature  at  which  the  reaction  is  carried  out ;  the  presence 
of  sulphuric  acid  facilitates  nitration  for  reasons  already 
mentioned.  The  chemical  properties  of  the  nitro-naphthalenes 
are  in  all  respects  similar  to  those  of  the  nitre-benzenes. 

a-Nitronaphthalene,  C10HK-N02,  is  best  prepared  in  small 
quantities  by  dissolving  naphthalene  in  acetic  acid, 
adding  concentrated  nitric  acid,  and  then  heating  on 
a  water-bath  for  half  an  hour;  the  product  is  poured  into 
water,  and  the  nitronaphthalene  purified  by  recrystallis- 
ation  from  alcohol.  On  the  large  scale  it  is  prepared  by 
treating  naphthalene  with  nitric  and  sulphuric  acids,  the 
method  being  similar  to  that  employed  in  the  case  of  nitro- 
benzene. It  crystallises  in  yellow  prisms,  melts  at  61°,  and 
boils  at  304° ;  on  oxidation  with  nitric  acid,  it  yields  nitro- 
phthalic  acid  (p.  445). 

^-Nitronaphthalene  is  not  formed  on  nitrating  naphthalene, 
but  it  may  be  prepared  by  dissolving  /3-nitro-a-naphthylarnine 
(a  compound  obtained  on  treating  a-naphthylamine  with 
dilute  nitric  acid)  in  an  alcoholic  solution  of  hydrogen  chloride, 
adding  finely-divided  sodium  nitrite,  and  then  heating  the 
solution  of  the  diazo-compound  (compare  p.  371), 

C10H6(N09).N:NC1  +  C0H5-OH  -= 

C10Hr-:NT02  +  N2  +  HC1  +  C2H40. 

It  crystallises  in  yellow  needles,  melting  at  79°. 

The  amido-derivatives  of  naphthalene  are  very  similar  in 
properties  to  the  corresponding  benzene  derivatives,  except 
that  even-the  monamido-compounds  are  crystalline  solids ;  they 
have  a  neutral  reaction  to  litmus,  and  yet  are  distinctly  basic 
in  character,  since  they  neutralise  acids,  forming  salts,  which, 
however,  are  decomposed  by  the  hydroxides  and  carbonates 
of  the  alkalies.  These  amido-compounds,  moreover,  may  be 


452  NAPHTHALENE    AND    ITS    DERIVATIVES. 

converted  into  diazo-com  pounds,  amidoazo-compounds,  &c., 
by  reactions  similar  to  those  employed  in  the  case  of  the 
amido-benzenes,  and  many  of  the  substances  obtained  in  this 
way,  as  well  as  the  amido-compounds  themselves,  are  exten- 
sively employed  in  the  manufacture  of  dyes. 

a-Naphthylamine,  C10H7-]STH2,  may  be  obtained  by  heating 
a-naphthol  with  ammonio-zinc  chloride,  or  ammonio-calcium 
chloride,* 

C10HrOH  +  NH3  =  C10Hrira2  +  H20, 

but  it  is  best  prepared  by  reducing  a-nitronaphthalene  with 
iron-filings  and  acetic  acid, 


It  is  a  colourless,  crystalline  substance,  melting  at  50°,  and 
boiling  at  300°  ;  it  has  a  disagreeable  smell,  turns  red  on 
exposure  to  the  air,  and  its  salts  give  a  blue  precipitate  with 
ferric  chloride  and  other  oxidising  agents.  On  oxidation 
with  a  boiling  solution  of  chromic  acid,  it  is  first  converted 
into  a-naphthaquinone  (p.  455),  and  then  into  phthalic  acid. 

/2-Naphthylamine  is  not  prepared  from  /3-nitronaphthalene 
(as  this  substance  is  itself  only  obtained  with  difficulty),  but 
from  /3-naphthol,  as  described  in  the  case  of  the  a-compound. 
It  crystallises  in  colourless  plates,  melts  at  112°,  and  boils  at 
294°  ;  it  differs  markedly  from  a-naphthylamine  in  having 
only  a  faint  odour,  and  its  salts  give  no  colouration  with 
ferric  chloride.  On  oxidation  with  potassium  permanganate, 
it  yields  phthalic  acid. 

The  two  naphthols,  or  monohydroxy-derivatives  of 
naphthalene,  correspond  with  the  monohydric  phenols,  and 

*  Prepared  by  passing  ammonia  over  anhydrous  zinc  or  calcium  chloride. 
These  compounds  decompose  when  heated,  evolving  ammonia,  and  are, 
therefore,  conveniently  employed  in  many  reactions  requiring  the  pres- 
ence of  ammonia  at  high  temperatures  ;  the  zinc  or  calcium  chloride 
resulting  from  their  decomposition  also  favours  the  reaction  in  those  cases 
in  which  water  is  formed,  as  both  substances  are  powerful  dehydrating 
agents.  Ammonium  acetate  may  be  employed  for  a  similar  purpose,  as 
it  dissociates  at  comparatively  low  temperatures,  but  its  action  is  less 
energetic. 


NAPHTHALENE   AND    ITS    DERIVATIVES.  453 

are  compounds  of  considerable  importance,  as  they  are  exten- 
sively employed  in  the  colour  industry.  They  both  occur  in 
coal-tar,  but  only  in  small  quantities,  and  are,  therefore, 
prepared  either  by  diazotising  the  corresponding  naphthyl- 
amines, 

C10HrXH2 *  CloHrN:NCl  -— »  C10HrOH, 

or  by  fusing  the  corresponding  sulphonic  acids  with  potash 
(compare  p.  387), 

C10HrS08K  +  KOH  .  C10H7-OH  +  K2S08. 
Their  properties  are,  on  the  whole,  very  similar  to  those  of  the 
phenols,  and,  like  the  latter,  they  dissolve  in  alkalies,  yield- 
ing metallic  derivatives,  which  are  decomposed  by  carbon 
dioxide ;  the  hydrogen  of  the  hydroxyl-group  in  the  naph- 
thols  may  also  be  displaced  by  an  acetyl-group  or  by  an  alkyl- 
group,  just  as  in  phenols,  and  on  treatment  with  pentachloride 
or  pentabromide  of  phosphorus,  a  halogen  atom  is  substituted 
for  the  hydroxyl-group.  The  naphthols  further  resemble  the 
phenols  in  giving  a  colour  reaction  with  ferric  chloride. 

In  a  few  respects,  however,  there  are  certain  differences  between 
the  chemical  properties  of  the  naphthols  and  phenols,  inasmuch  as 
the  hydroxyl-groups  in  the  former  more  readily  undergo  change  ; 
when,  for  example,  anaphthol  is  heated  with  ammonio-zinc  chloride 
at  250°,  it  is  converted  into  the  corresponding  amido-compound  (see 
above),  whereas  the  conversion  of  phenol  into  aniline  requires  a 
temperature  of  300-350°,  other  conditions  remaining  the  same. 
Again,  when  a  naphthol  is  heated  with  an  alcohol  and  hydrogen 
chloride,  it  is  converted  into  an  alkyl-derivative,  whereas  alkyl- 
derivatives  of  phenols  cannot,  as  a  rule,  be  obtained  in  this  way  ;  in 
this  respect,  the  naphthols  form,  as  it  were,  a  connecting-link 
between  the  phenols  and  the  alcohols. 

a-Naphthol,  C10H7-OH,  is  formed,  as  previously  stated 
(p.  447),  on  boiling  phenylisocrotonic  acid  with  water,  an 
important  synthesis,  which  proves  that  the  hydroxyl-group  is 
in  the  a-position  ;  it  is  prepared  from  a-naphthylamine  or  from 
naphthalene-a-sulphonic  acid  (see  above).  It  is  a  colourless, 
crystalline  substance,  melting  at  94°,  and  boiling  at  280° ;  it 
has  a  faint  smell,  recalling  that  of  phenol,  and  it  dissolves 


454  NAPHTHALENE    AND    ITS    DERIVATIVES. 

freely  in  alcohol  and  ether,  but  is  only  sparingly  soluble  in 
hot  water.  Its  aqueous  solution  gives  with  ferric  chloride  a 
violet,  flocculent  precipitate,  consisting  probably  of  an  iron 
compound  of  a-dinaphthol,  OH-C10H6-C10H6-OH,  an  oxida- 
tion product  of  the  naphthol. 

a-Naphthol,  like  phenol,  is  very  readily  acted  on  by  nitric 
acid,  yielding  a  r^'mYro-derivative,  C10H5(N02)2-OH,  which 
crystallises  in  yellow  needles,  and  melts  at  138°;  this  nitro- 
compound,  like  picric  acid,  has  a  much  more  strongly 
marked  acid  character  than  the  hydroxy-compound  from 
which  it  is  derived,  and  decomposes  carbonates,  forming  deep- 
yellow  salts  which  dye  silk  a  beautiful  golden  yellow;  its  sodium 
derivative,  C10H5(N02)2-ONa  +  H20,  is  known  commercially 
as  Martins'  yellow,  or  naplitlialene  yellow.  Another  dye  obtained 
from  a-naphthol  is  naphthol  yellow  (p.  527),  the  potassium  salt 
of  dinitro-a-naphtholsulphonic  acid,  C10H4(N02)2(OK)-S03K; 
the  acid  itself  is  manufactured  by  nitrating  a-naphtholtri- 
sulphonic  acid  (prepared  by  heating  a-naphthol  with 
anhydrosulphuric  acid),  in  which  process  two  of  the  sulphonic 
groups  are  displaced  by  nitro-groups. 

/^-Naphthol,  prepared  by  fusing  naphthalene-/3-sul phonic 
acid  with  potash  (p.  453),  melts  at  122°,  and  boils  at 
285° ;  it  is  a  colourless,  crystalline  compound,  readily  soluble 
in  hot  water,  and  like  the  a-derivative,  it  has  a  faint 
phenol-like  smell.  Its  aqueous  solution  gives,  with  ferric 
chloride,  a  green  colouration  and  a  flocculent  precipitate  of 
/3-dinaphthol,  OH.C10H6.C10H6.OH. 

Sulphonic  Acids. — Perhaps  the  most  important  derivatives 
of  naphthalene,  from  a  commercial  point  of  view,  'are  the 
various  mono-  and  di-sulphonic  acids,  which  are  obtained 
from  the  hydrocarbon  itself,  from  the  naphthyla mines,  and 
from  the  naphthols,  many  of  these  compounds  being  used  in 
large  quantities  in  the  manufacture  of  dyes.  It  would  be 
impossible  to  give  here  even  the  names  of  the  very  numerous 
compounds  of  this  class,  but  some  indication  of  their 
properties  may  be  afforded  by  the  following  statements : 


NAPHTHALENE    AND    ITS    DERIVATIVES.  455 

Naphthalene  is  readily  sulplionated,  yielding  two  mono- 
sul phonic  acids,  C10H7-S03H,  namely,  the  a-  and  /3-com pounds, 
both  of  which  are  formed  when  the  hydrocarbon  is  heated 
with  concentrated  sulphuric  acid  at  80° ;  if,  however,  the 
operation  be  carried  out  at  160°,  only  the  /3-acid  is  obtained, 
because  at  this  temperature  the  a-acid  is  converted  into  the 
/3-acid  by  intramolecular  change,  just  as  phenol-o-sulphoriic 
acid  is  transformed  into  the  p-acid  by  heating.  The  two 
naphthalenesulphonic  acids  are  crystalline  hygroscopic  sub- 
stances, and  show  all  the  characteristic  properties  of  acids  of 
this  class. 

Di-  and  tri-sulphonic  acids  may  be  obtained  by  strongly 
heating  naphthalene  with  sulphuric  or  anhydrosulphuric  acid. 

Fourteen  isomeric  naphthyla?ninemonosidpJtonic  acids, 
C10H6(XH2)-S03H,  may  theoretically  be  obtained — namely, 
seven  from  a-naphthylamine,  and  seven  from  the  /3-base ;  as  a 
matter  of  fact,  nearly  all  these  acids  are  known.  One  of  the 
most  important,  perhaps,  is  l:4-naphthylaminemonosulphonic 
acid,  or  naphthionic  acid,  which  is  the  sole  product  of  the  action 
of  sulphuric  acid  on  a-naphthylamine ;  it  is  a  crystalline 
compound,  very  sparingly  soluble  in  cold  water,  and  is  used 
in  the  manufacture  of  Congo- red  (p.  526),  and  other  dyes. 

The  naphtholmonosulphonic  acids  correspond  in  number 
with  the  naphthylaminemonosulphonic  acids,  and  are  also 
extensively  used  in  the  colour  industry. 

a-Naphthaquinone,  C10H602,  is  a  derivative  of  naphthalene 
corresponding  with  (benzo)quinone,  and,  like  the  latter,  it  is 
formed  on  oxidising  various  mono-  and  di-substitution  products 
of  the  hydrocarbon  with  sodium  bichromate  and  sulphuric 
acid,  but  only  those  in  which  the  substituting  groups  occupy 
thea-positions;  a-naphthylamine,  1 : 4-amidonaphthol,  and  1:4- 
diamidonaphthalene,  for  example,  may  be  employed.  As  a 
rule,  however,  naphthalene  itself  is  oxidised  with  a  boiling 
solution  of  chromic  acid  in  acetic  acid  (a  method  not 
applicable  for  the  preparation  of  quinone  from  benzene),  as 
the  product  is  then  easily  obtained  in  a  state  of  purity. 


456  NAPHTHALENE    AND    ITS    DERIVATIVES. 

a-Naphthaquinone  crystallises  from  alcohol  in  deep- 
yellow  needles,  melting  at  125°;  it  resembles  quinone  in 
colour,  in  having  a  curious  pungent  smell,  and  in  being 
very  volatile,  subliming  readily  even  at  100°,  and  distilling 
rapidly  in  steam.  Like  quinone,  moreover,  it  is  readily  re- 
duced by  sulphurous  acid,  yielding  1 : 4-dihydroxynaphthalene, 
C10H6(OH)2,  just  as  quinone  yields  hydroquinoue  (p.  399). 
This  close  similarity  in  properties  clearly  points  to  a  similarity 
in  constitution,  so  that  a-naphthaquinone  may  be  represented 
by  the  formula, 


for  reasons  similar  to  those  stated  more   fully  in  the  case  of 
quinone. 

/?-Naphthaquinone,  C10H602,  isomeric  with  the  a-compound, 
is  formed  when  a-amido-/?-naphthol  is  oxidised  with 
potassium  bichromate  and  dilute  sulphuric  acid,  or  with 
ferric  chloride  ;  it  crystallises  in  red  needles,  decomposes  at 
about  115°  without  melting,  and  on  reduction  with  sulphur- 
ous acid,  is  converted  into  1 : 2-dihydroxynaphthalene.  It 
differs  from  a-naphthaquinone  and  from  quinone  in  colour, 
in  having  no  smell,  and  in  being  non-volatile,  properties 
which,  though  apparently  insignificant,  are  really  of  some 
importance,  as  showing  the  difference  between  orfho-quiuoues 
and  jpara-quinones ;  the  latter  are  generally  deep-yellow, 
volatile  compounds,  having  a  pungent  odour,  whereas  the 
former  are  red,  non-volatile,  and  odourless.  /3-Naphtha- 
quinone  is  an  example  of  an  ortho-quinone,  and  its  consti- 
tution may  be  represented  by  the  formula, 


The    above    description    of    some    of    the   more    important 


NAPHTHALENE    AND    ITS    DERIVATIVES.  457 

naphthalene  derivatives  will  be  sufficient  to  show  the  close 
relationship  which  these  compounds  bear  to  the  corresponding 
derivatives  of  benzene  ;  although  the  former  exist  in  a  larger 
number  of  isomeric  forms,  they  are,  as  a  rule,  prepared  by  the 
same  methods  as  their  analogues  of  the  benzene  series,  and 
resemble  them  closely  in  chemical  properties.  It  may,  in  fact, 
be  stated  as  a  general  rule,  that  all  general  reactions  and 
generic  properties  of  benzene  derivatives  are  met  with  again 
in  studying  naphthalene  derivatives. 


CHAPTER    XXXI. 

ANTHRACENE  AND  PHENANTHRENE. 

Anthracene,  C14H10,  is  a  hydrocarbon  of  great  commercial 
importance,  as  it  is  the  starting-point  in  the  manufacture  of 
alizarin,  the  colouring  matter  employed  in  producing  Turkey- 
red  dye  ;  it  is  prepared  exclusively  from  coal-tar.  The  crude 
mixture  of  hydrocarbons  and  other  substances  known  as  '50 
per  cent,  anthracene  '  (p.  298)  is  first  distilled  with  one-third  of 
its  weight  of  potash  from  an  iron  retort  ;  the  distillate,  which 
consists  almost  entirely  of  anthracene  and  phenanthrene,  is 
then  treated  with  carbon  bisulphide,  when  the  phenanthrene 
dissolves,  leaving  the  anthracene,  which  is  further  purified  by 
crystallisation  from  benzene. 

Crude  anthracene  contains  considerable  quantities  of  carbazole, 


/NH,  a  colourless,  crystalline  substance,  melting  at  238°,  and 

PR/ 

L6n4 

boiling  at  355°.      On   treatment  with  potash,   this   substance  is 

CeH4\ 

converted  into  a  potassium  derivative,    I         /NK,  which  remains 

P   TT  / 

U6rL4 

in  the  retort,  or  is  decomposed  on  subsequent  distillation  ;  many 
other  impurities,  which  cannot  readily  be  separated  by  crystallisa- 
tion, are  also  got  rid  of  in  this  way. 

Anthracene  crystallises  from  benzene  in  colourless,  lustrous 


458  ANTHRACENE   AND    PHENANTHRENE. 

plates,  which  show  a  beautiful  blue  fluorescence ;  it  melts  at 
213°,  boils  at  about  3GO°,  and  dissolves  freely  in  boiling 
benzene,  but  is  only  sparingly  soluble  in  alcohol  and  ether. 
On  mixing  saturated  alcoholic  solutions  of  anthracene  and 
picric  acid,  anthracene  picrate,  C14H10,C6H2£N"02)3-OH,  is 
deposited  in  ruby-red  needles,  which  melt  at  138°;  this 
compound  is  resolved  into  its  components  when  treated  with 
a  large  quantity  of  alcohol  (distinction  from  phenanthrene 
picrate,  p.  468). 

Constitution. — The  behaviour  of  anthracene  towards  chlorine 
and  bromine  is,  on  the  whole,  similar  to  that  of  benzene  and 
naphthalene — that  is  to  say,  it  yields  additive  or  substitution 
products  according  to  the  conditions  employed ;  towards 
concentrated  sulphuric  acid,  also,  it  behaves  like  other  aromatic 
compounds,  and  is  converted  into  sulphonic  acids  by  substitu- 
tion. "When  treated  with  nitric  acid,  however,  instead  of 
yielding  a  nitro-derivative,  as  was  to  be  expected  from  the 
molecular  formula  of  the  hydrocarbon  (which,  from  the 
relatively  small  proportion  of  hydrogen,  clearly  indicates  the 
presence  of  one  or  more  closed  chains),  it  is  oxidised  to  anthra- 
quinone,  C14H802,  two  atoms  of  hydrogen  being  displaced  by 
two  atoms  of  oxygen ;  this  change  always  takes  place,  even 
when  dilute  nitric  acid,  or  some  other  oxidising  agent,  is 
employed,  and  as  it  is  closely  analogous  to  that  which  occurs  in 
the  conversion  of  naphthalene,  C10H8,  into  a-naphthaquinone, 
C10H602  (p.  455),  it  is  an  indication  of  the  presence  of  a  closed- 
chain,  oxidation  processes  of  this  kind  (namely,  the  substitu- 
tion of  oxygen  for  an  equal  number  of  hydrogen  atoms)  being 
unknown  in  the  case  of  fatty  (open-chain)  substances. 
Another  highly  important  fact,  owing  to  its  bearing  on  the 
constitution  of  anthracene,  is  this,  that,  although  the  hydro- 
carbon and  most  of  its  derivatives  are  resolved  into  simpler 
substances  only  with  very  great  difficulty,  when  this  does 
occur,  one  of  the  products  is  always  some  benzene  derivative, 
usually  phthalic  acid. 

Now,   if  the  molecule  of  anthracene  contained    only  one 


ANTHRACENE  AND  PHENANTHRENE.  459 

benzene  nucleus,  or  even  if,  like  naphthalene,  it  contained 
two  condensed  nuclei,  there  would  still  be  certain  carbon 
and  hydrogen  atoms  to  be  accounted  for,  and  this  could 
only  be  done  by  assuming  the  presence  of  unsaturated  side- 
chains  ;  as,  however,  all  experience  has  shown  that  such 
side-chains  in  benzene  and  in  naphthalene  are  oxidised  to 
carboxyl  (compare  p.  327)  with  the  utmost  facility,  it  is  impos- 
sible to  accept  the  assumption  of  their  presence  in  anthracene, 
a  compound  which  is  always  oxidised  to  the  neutral  substance 
anthraquinone,  without  loss  of  carbon.  Arguments  of  this 
kind  lead,  therefore,  to  only  one  conclusion — namely,  that  the 
molecule  of  anthracene  is  composed  only  of  combined  or 
condensed  nuclei ;  as,  moreover,  the  hydrocarbon  may  be 
indirectly  converted  into  phthalic  acid,  it  must  be  assumed 
that  two  of  these  nuclei  are  condensed  together  in  the 
o-position,  as  in  naphthalene. 

If,  now,  an  attempt  be  made  to  deduce  a  constitutional 
formula  for  anthracene  on  this  basis,  and  it  be  further  assumed 
that  all  the  closed-chains  are  composed  of  six  carbon 
atoms,  as  in  naphthalene,  the  following  formulae  suggest  them- 
selves as  the  most  probable, 

CH  CH 


CH  CH1- 

v^  , i  ,       -s^  ^  -^w 

CH 


'•  II. 

although,  of  course,  neither  could  be  accepted  as  final  without 
further  evidence. 

Experience  has  shown,  however,  that  formula  i.  must  be  taken 
as  representing  the  constitution  of  anthracene  (formula  n. 
expressing  that  of  phenanthrene,  p.  468),  because  it  accounts 
satisfactorily  for  all  known  facts,  amongst  others,  for  a  number 
of  important  syntheses  of  the  hydrocarbon  (see  below),  for  the 


460          ANTHRACENE  AND  PHENANTHRENE. 

relation  of  anthracene  to  anthraquinone,  and  for  the  isomerism 
of  the  anthracene  derivatives.  It  is,  nevertheless,  just  as  diffi- 
cult to  determine  and  to  express  the  actual  disposition  of  the 
fourth  affinity  of  each  carbon  atom  in  anthracene,  as  in  the  cases 
of  benzene  and  naphthalene  ;  as,  however,  there  are  reasons  for 
supposing  that  the  state  of  combination  of  the  two  central  CH 
groups  (that  is,  those  which  form  part  of  the  central  nucleus 
only)  is  different  from  that  of  all  the  others  (inasmuch  as  they 
are  generally  attacked  first),  and  that  the  two  carbon  atoms 
of  these  groups  are  directly  united,  the  above  formula  (i.)  is 
usually  written 


/CH\ 

or     C6H4<J     >C6H4, 

'  XCH/ 


the  disposition  of  the  fourth  affinities  of  the  carbon  atoms 
in  the  two  C6H4<d  groups  being  taken  to  be  the  same  as 
in  the  centric  formula  for  benzene.* 

Anthracene  may  be  obtained  synthetically  in  various  ways. 
It  is  produced  when  benzyl  chloride  is  heated  with  aluminium 
chloride, 

PIT 
3C6H5.CH2C1  =  C6H4<  ,     >C6H4  +  CliH5.CH3  +  3HCI, 

Oil 

the  hydranthracene  (p.  461)  which  is  formed  as  an  interme- 
diate product, 

4  +  2HC1> 

being  converted  into  anthracene  by  loss  of  hydrogen,  which 
reduces  part  of  the  benzyl  chloride  to  toluene,  as  shown  in 
the  first  equation.  Anthracene  is  also  formed,  together  with 
hydranthracene  and  phenanthrene  (p.  469),  when  0?*/7j0-bromo- 

*  The  letters  or  numbers  serve  to  denote  the  constitution  of  the  anthra- 
cene derivatives  (p.  461). 


ANTHRACENE    AND    PHENANTHRENE.  461 

benzyl  bromide  (prepared  by  brominating   boiling    o-bromo- 
toluene,  C6H4Br-CH3)  is  treated  with  sodium, 


here,  again,  hydranthracene  is  the  primary  product,  and  from 
it  anthracene  is  formed  by  loss  of  hydrogen. 

Another  interesting  synthesis  may  be  mentioned  —  namely, 
the  formation  of  anthracene  on  treating  a  mixture  of  tetra- 
bromethane  and  benzene  with  aluminum  chloride, 

H     Br(?HBr  4.  H^r  P     r  H  /?HV  fl 

H^H4  =  C6H*/  6    4 


BrCHBr 

All  these  methods  of  formation  are  accounted  for  in  a  simple 
manner  with  the  aid  of  the  above  constitutional  formula,  the 
last  one  especially  indicating  that  the  two  central  carbon 

atoms  are  directly  united;    the  formula    C6H4<^  i^    ~^>C6H4 

CH 

will,  therefore,  be  employed  in  describing  the  anthracene 
derivatives. 

Isomerism  of  Anthracene  Derivatives. — Further  evidence 
in  support  of  the  above  constitutional  formula  is  afforded 
by  the  study  of  the  isomerism  of  the  substitution  products  of 
anthracene,  although,  in  most  cases,  all  the  isomerides  theo- 
retically possible  have  not  yet  been  prepared. 

When  one  atom  of  hydrogen  is  displaced,  three  isomerides 
may  be  obtained,  since  there  are  three  hydrogen  atoms  (a,/3,y), 
all  of  which  are  differently  situated  relatively  to  the  rest  of 
the  molecule;  these  mono-substitution  products  are  usually 
distinguished  by  the  letters  a,  /?,  y,  according  to  the  position 
of  the  substituent  (compare  formula  p.  460).  When  two 
atoms  of  hydrogen  are  displaced  by  similar  atoms  or  groups, 
fifteen  isomeric  di-substitution  products  may  be  obtained. 

Hydranthracene,  C6H4<£^£>C6H4,  a  substance  of  little 
importance,  is  formed  on  reducing  anthracene  with  boiling 


462          ANTHRACENE  AND  PHENANTHRENE. 

concentrated  hydriodic  acid,  or  with  sodium  amalgam.  It 
is  a  colourless,  crystalline  compound,  melting  at  106-108°, 
and  when  heated  with  sulphuric  acid,  it  is  converted  into 
anthracene,  the  acid  being  reduced  to  sulphur  dioxide. 

Anthracene  dicliloride,  C6H4<\TT,^>C6H4,    like  hydran- 


thracene,  is  an  additive  product  of  the  hydrocarbon  ;  it  is 
obtained  when  chlorine  is  passed  into  a  cold  solution  of 
anthracene  in  carbon  bisulphide,  whereas  at  100°  substitution 

CC1 

takes   place,  monochloranthracene,    C6H4<^  I    ^/CgH^,    and 

CH 


dichloranthracene,  C6H4<^  I     ^CgH^,   being  formed  ;   these 

substitution  products  crystallise  in  yellow  needles,  melting 
at  103°  and  209°  respectively,  and  they  are  both  converted 
into  anthraquinone  on  oxidation,  a  fact  which  shows  the 
positions  of  the  chlorine  atoms. 

Anthraquinone,  C6H4<^    />C6H4,  is  formed,  as  already 

mentioned,  on  oxidising  anthracene  with  chromic  or  nitric 
acid.  It  is  conveniently  prepared  by  dissolving  anthracene 
(1  part)  in  boiling  glacial  acetic  acid,  and  gradually  adding  a 
concentrated  solution  of  chromic  acid  (2  parts)  in  glacial 
acetic  acid.  As  soon  as  oxidation  is  complete,  the  product  is 
allowed  to  cool,  and  the  anthraquinone,  which  separates  in 
long  needles,  is  collected  and  purified  either  by  sublimation 
or  by  recrystallisation  from  acetic  acid. 

Anthraquinone  is  manufactured  by  oxidising  finely-divided  '50 
per  cent,  anthracene,'  suspended  in  water,  with  the  calculated 
quantity  of  sodium  bichromate  and  sulphuric  acid.  The  crude 
anthraquinone  is  collected  on  a  filter,  washed,  dried,  and  heated 
at  100°  with  2-3  parts  of  concentrated  sulphuric  acid,  by  which 
means  the  impurities  are  converted  into  soluble  sulphonic  acids, 
whereas  the  anthraquinone  is  not  acted  on.  The  almost  black 
product  is  now  allowed  to  stand  in  a  damp  place,  when  the  anthra- 
quinone gradually  separates  in  crystals  as  the  sulphuric  acid 


ANTHRACENE  AND  PHENANTHRENE.          463 

becomes  dilute  ;  water  is  then  added,  and  the  anthraquinone  col- 
lected, washed,  and  dried  and  sublimed. 

Anthraquinone  may  be  produced  synthetically  by  treating 
a  solution  of  phthalic  anhydride  (p.  426)  in  benzene,  with  a 
strong  dehydrating  agent,  such  as  aluminium  chloride,  the 
reaction  taking  place  in  two  stages  ;  o-benzoylbenzoic  acid  is 
first  produced, 

° 


o-Benzoylbenzoic  Acid. 

but  by  the  further  action  of  the  aluminium  chloride  (or  when 
treated  with  sulphuric  acid),  this  substance  is  converted  into 
anthraquinone  with  loss  of  1  molecule  of  water, 


A  BAB 

Anthraquinone  contains,  therefore,  two  C6H4<^  groups,  united 
by  two  C0<^  groups. 

That  the  two  C0<^  groups  occupy  the  o-position  in  the  one 
benzene  ring  (A)  is  known,  because  they  do  so  in  phthalic  acid  ;  that 
they  occupy  the  o-position  in  the  second  benzene  ring  (B)  has  been 
proved,  as  follows  :  When  bromophthalic  anhydride  is  treated 
with  benzene  and  aluminium  chloride,  bromobenzoylbenzoic  acid 
is  produced,  and  this,  when  treated  with  sulphuric  acid,  yields 
bromanthraquinone, 

C6H3B<C°^C6H5  =  C6H3B,<££>C6H4  +  H2O. 

A  BAB 

The  formation  of  this  substance  from  bromophthalic  acid  proves, 
as  before,  that  the  two  CO\  groups  are  united  to  the  ring  A  in 
the  o-position. 

Now,  when  bromanthraquinone  is  heated  with  potash  at  160°,  it 

is  converted   into   hydroxyanthraquinone,    C6H3(OH)<^Q>C6H4, 

A  B 

and   this,    on    oxidation  with    nitric    acid,   yields   phthalic  acid, 
>C6H4,  the  group  A  being  destroyed  ;  therefore  the  two  CO< 


groups  are  attached  to  B,  as  well  as  to  A,  in  the  o-position,  and 
therefore  anthraquinone  has  the  constitution  represented  above, 


464          ANTHRACENE  AND  PHENANTHRENE. 

a  conclusion  which  affords  strong   support  to  the   above  views 
regarding  the  constitution  of  anthracene. 

Anthraquinone  crystallises  from  glacial  acetic  acid  in  pale- 
yellow  needles,  melts  at  277°,  and  sublimes  very  readily  at 
higher  temperatures  in  long,  sulphur-yellow  prisms;  it  is 
exceedingly  stable,  and  is  only  with  difficulty  attacked  by 
oxidising  agents,  by  sulphuric  acid,  or  by  nitric  acid.  In  all 
those  properties  which  are  connected  with  the  presence  of 
the  two  carbonyl-groups,  anthraquinone  resembles  the  aromatic 
ketones  much  more  closely  than  the  quinones.  It  has  no 
smell,  is  by  no  means  readily  volatile,  and  is  not  reduced 
when  treated  with  sulphurous  acid  ;  unlike  quinone,  there- 
fore, it  is  not  an  oxidising  agent.  When  treated  with  more 
powerful  reducing  agents,  however,  it  is  converted  into 

PO 
oxanthranol,  C6H4<^j^Qjj,p>C6H4,  one  of  the  CCX^  groups 

becoming  ^>CH-OH,  just  as  in  the  reduction  of  ketones;  on 
further  reduction  the  other  C0<^  group  undergoes  a  similar 

change,  but  the  product,  CeH^^6114'  loses  one 


molecule  of  water,  yielding  anthranol,  C6H4<^i     ^^^C6H4, 

which  is  finally  reduced  to  hydra  nth  racene  ;  when  anthra- 
quinone is  distilled  with  zinc-dust,  anthracene  is  produced. 
Anthraquinone  is  only  slowly  acted  on  by  ordinary  sulphuric 
acid  even  at  250°,  yielding  anthraquinone-/?-monosulphonic 

acid,  C6H4<^p^^>C6H3-S03H  ;  but  when  heated  with  a  large 

excess  of  anhydrosulphuric  acid  at  160-170°,  it  yields  a 
mixture  of  disulphonic  acids,  C14H602(S03H)2. 

Sodium  anthraquinone-monosulphonate,  which  is  used  in  such 
large  quantities  in  the  manufacture  of  alizarin  (see  below),  is  pre- 
pared by  heating  anthraquinone  with  an  equal  weight  of  anhydro- 
sulphuric acid  (containing  50  per  cent,  of  SO3)  in  enamelled  iron 
pots  at  160°.  The  product  is  diluted  with  water,  filtered  from  un- 
changed anthraquinone,  and  neutralised  with  soda;  on  cooling, 
sparingly  soluble  sodium  anthraquinone-monosulphonate  separates 


ANTHRACENE  AND  PHENANTHRENE.          465 

in  glistening  plates,  and  is  collected  in  filter-presses.  The  more 
soluble  sodium  salts  of  the  anthraquinone-disulphonic  acids,  which 
are  always  formed  at  the  same  time,  remain  in  solution. 

Test  for  Anthraquinone. — When  a  trace  of  finely-divided 
anthraquinone  is  mixed  with  dilute  soda,  a  little  zinc-dust 
added,  and  the  mixture  heated  to  boiling,  an  intense  red 
colouration  is  produced,  but  on  shaking  in  contact  with  air, 
the  solution  is  decolourised ;  in  this  reaction  oxanthranol  is 
formed,  and  this  substance  dissolves  in  the  alkali,  forming  a 
deep-red  solution  ;  on  shaking  with  air,  however,  it  is  oxidised 
to  anthraquinone,  which  separates  as  a  white  flocculent 
precipitate. 

Alizarin,  C6H4<^>C6H2(OH)2,  or  a/?-dihydroxyanthra- 

quinone,  occurs  in  madder  (the  root  of  Rubia  tinctorum\  a  sub- 
stance which  has  been  used  from  the  earliest  times  for  dyeing 
purposes,  and  which  owes  its  tinctorial  properties  to  two  sub- 
stances, alizarin  and  purpurin  (see  below),  both  of  which  are 
present  in  the  root  in  the  form  of  glucosides.  Ruberythric 
acid,  the  glucoside  of  alizarin,  is  decomposed  when  boiled 
with  acids,  or  when  the  madder  extract  is  allowed  to  undergo 
fermentation,  with  formation  of  alizarin  and  two  molecules 
of  dextrose, 

C26H280U  +  2H20  =  CUH804  +  2C6H1209. 

Ruberythric  Acid.  Alizarin. 

A  dye  of  such  great  importance  as  alizarin  naturally  attracted 
the  attention  of  chemists,  and  many  attempts  were  made  to 
prepare  it  synthetically.  This  was  first  accomplished  in  1868 
by  Graebe  and  Liebermann,  who  found  that  alizarin  could 
be  produced  by  fusing  a/3-dibromanthraquinone*  with  potash, 

C6H4<^>C6H2Br2  +  2KOH  = 

C6H4<£a>C6H2(OH)2  +  2KBr, 

but  the  process  was  not  a  commercial  success. 

*  Obtained  by  heating  anthraquinone  with  bromine  and  a  trace  of  iodine 
in  a  sealed  tube  at  160°. 


466          ANTHRACENE  AND  PHENANTHRENE. 

At  the  present  day,  however,  the  madder  root  is  no  longer 
used,  and  the  whole  of  the  alizarin  of  commerce  is  made 
from  (coal-tar)  anthracene  in  the  following  manner  : 

Anthracene  is  first  oxidised  to  anthraquinone,  and  the 
latter  is  converted  into  anthraquinone-/3-sulphonic  acid  by  the 
method  already  described  (p.  464)  ;  the  sodium  salt  of  this  acid 
is  then  fused  with  soda  and  a  little  potassium  chlorate,  and 
is  thus  converted  into  the  sodium  derivative  of  alizarin, 

3.S03Na  +  3NaOH  +  0  = 

ro 


+  2H20  +  Na2S08; 

from  this  sodium  salt  the  colouring  matter  itself  is  obtained 
by  adding  acid. 

When  anthraquinonesulphonic  acid  is  fused  with  soda,  the 
-S03H  group  is  displaced  by  -OH  in  the  usual  manner,  but  the 
hydroxy  anthraquinone  thus  produced  is  very  readily  converted  into 
alizarin  by  the  further  action  of  the  soda,  part  of  it  being  reduced 
to  anthraquinone, 

2C6H4<C°>C6H3(OH)  = 

Hydroxyanthraqninone. 

C6H4<g°>C6H2(OH)2  +  C6H4<g°>C6H4. 

This  regeneration  of  anthraquinone,  and  consequent  diminished 
yield  of  alizarin,  is  prevented  by  the  addition  of  the  oxidising 
agent  (KC103)  ;  the  operation  is  usually  conducted  as  follows  : 

Sodium  anthraquinonesulphonate  (100  parts)  is  heated  in  a 
closed  iron  cylinder,  fitted  with  a  stirrer,  with  soda  (300  parts) 
and  potassium  chlorate  (14  parts),  for  two  days  at  180°.  The  dark- 
violet  product,  which  consists  of  the  sodium  salt  of  alizarin,  is 
dissolved  in  water,  the  solution  filtered  if  necessary,  and  the  alizarin 
precipitated  by  the  addition  of  hydrochloric  acid.  The  yellowish 
crystalline  precipitate  is  collected  in  filter-presses,  washed  well  with 
water,  and  sent  into  the  market  in  the  form  of  a  10  or  20  per  cent. 
paste.  From  this  product  alizarin  is  obtained  in  a  pure  state  by 
vecrystallisation  from  toluene,  or  by  sublimation. 

Alizarin  crystallises  and  sublimes  in  dark-red  prisms, 
which  melt  at  282°,  and  are  almost  insoluble  in  water,  but 


ANTHRACENE  AND  PHENANTHRENE.          467 

moderately  soluble  in  alcohol.  It  is  a  dihydroxy-derivative 
of  anthraquinone,  and  has  therefore  the  properties  of  a 
dihydric  phenol;  it  dissolves  in  potash  and  soda,  forming 

metallic  derivatives  of  the  type  C6H4<^p^^>C6H2(OM)2,  which 

are  soluble  in  water,  yielding  intensely  reddish-violet  solutions. 
With  acetic  anhydride  it  gives  a  diacetate,  C14H602(G2H302)2, 
melting  at  180°,  and  when  distilled  with  zinc-dust,  it  is 
reduced  to  anthracene. 

The  value  of  alizarin  as  a  dye  lies  in  the  fact  that  it  yields 
magnificently  coloured  insoluble  compounds  (called  'lakes') 
with  certain  metallic  oxides ;  the  ferric  compound,  for 
example,  is  violet  black,  the  lime  compound  blue,  and  the  tin 
and  aluminium  compounds  different  shades  of  red  (Turkey- 
red).  A  short  account  of  the  methods  used  in  dyeing  with 
alizarin  is  given  later  (p.  504). 

Constitution  of  Alizarin. — Alizarin  may  be  synthetically 
prepared  by  heating  a  mixture  of  phthalic  anhydride  and 
catechol  with  sulphuric  acid  at  150°, 

= 


As  catechol  is  o-dihydroxy benzene,  it  follows  that  the  two 
hydroxyl-groups  in  alizarin  must  be  in  the  o-position  to  one 
another,  and  this  substance  must,  therefore,  be  represented  by 
one  of  the  following  formulae  : 

CO  OH  CO 

^-^^^^^\  ^^^^ 

OH         p  7|^7          -I-  ]OH 

CO  CO 

I.  II. 

Xow  alizarin  yields  two  (a1  and  /31)  isomeric  mono-nitro- 
derivatives,   C6H4^Q>C6H(OH)2-N02,     both    of    which 


468  ANTHRACENE    AND    PHENANTHRENE. 

contain  the  nitro-group  in  the  same  nucleus  as  the  two 
hy  d  roxy  1-groups. 

The  constitution  of  alizarin  must,  therefore,  be  represented 
by  formula  i.,  as  a  substance  having  the  constitution  n.  could 
only  yield  one  such  iiitro-derivative,  and  this  formula  has 
been  shown  to  be  correct  in  many  other  ways  which  cannot 
be  discussed  here. 

Besides  alizarin,  several  other  dihydroxy-  and  also  trihydroxy- 
anthraquinones  have  been  obtained,  but  only  those  are  of  value  as 
dyes  which  contain  two  hydroxyl-groups  in  the  same  positions  as  in 
alizarin  ;  two  such  derivatives,  which  possess  very  valuable  dyeing 
properties,  may  be  mentioned. 

Purpurm,  C6H4<^  ^>C6H(OH)3,   or  ajScMrihydroxyanthraqui- 

none,  is  contained  in  madder  root,  in  the  form  of  a  glucoside,  and 
may  be  artificially  prepared  by  oxidising  alizarin  with  manganese 
dioxide  and  sulphuric  acid.  It  crystallises  in  deep-red  needles, 
melts  at  252°,  and  gives,  with  alumina  mordants,  a  much  yellower 
shade  of  red  than  alizarin,  and  is  now  used  on  the  large  scale  for 
the  production  of  brilliant  reds. 

Anthrapurpurin,  C6H3(OH)<^Q>C6H2<^(^,  is  isomeric  with 
purpurin,  and  is  manufactured  by  fusing  anthraquinone-disul- 
phouic  acid,  C6H3(S03H)<^>C6H3.SO3H,  with  soda  and  potass- 

ium  chlorate  (see  alizarin,  p.  466).  It  crystallises  in  yellowish- 
red  needles,  melts  at  330°,  and  is  very  largely  employed  in  dyeing 
yellow  shades  of  Turkey-red. 

Phenanthrene,  C14H10,  an  isomeride  of  anthracene,  is  a 
hydrocarbon  of  considerable  theoretical  interest,  although  it 
has  no  commercial  value.  It  occurs  in  large  quantities  in 
*  50  per  cent,  anthracene,'  from  which  it  may  be  extracted  as 
already  described  (p.  457).  The  resulting  crude  phenanthrene 
is  converted  into  the  picrate  (see  below),  which  is  h'rst  re- 
crystallised  from  alcohol,  to  free  it  from  anthracene  picrate, 
and  then  decomposed  by  ammonia,  the  hydrocarbon  being 
finally  purified  by  recrystallisation. 

Phenanthrene   crystallises  in  glistening  needles,   melts  at 


ANTHRACENE  AND  PHENANTHRENE.          469 

99°,  and  distils  at  about  340° ;  it  is  readily  soluble  in  alcohol, 
ether,  and  benzene.  When  oxidised  with  chromic  acid,  it  is 
first  converted  into  phenanihraquinone,  C14H802,  isomeric 
with  anthraquinone,  and  then  into  diphenic  acid,  C14H1004. 
This  acid  is  decomposed  on  distillation  with  lime,  yielding 
carbon  dioxide  and  diplienyl  (p.  340) ;  it  is  therefore  diplienyl- 
dicarboxylic  acid,  COOH-C6H4-C6H4-COOH,  and  its  formation 
from  phenanthrene  shows  that  the  latter  is  also  a  derivative 
of  diphenyl. 

Further  evidence  as  to  the  constitution  of  phenanthrene  is 
obtained  by  studying  its  methods  of  formation.  It  is  formed, 
for  example,  on  passing  o-ditolyl  (prepared  from  o-bromo- 
toluene  and  sodium)  or  stilbene  *  through  a  red-hot  tube,  and 
the  simplest  manner  of  expressing  these  two  reactions  is  the 
following : 


2H2 


C6H4  —  CH3 
C6H4-CH3 

o-Ditolyl. 

CH4—  CH 

1           II 
C6H4—  CH 

Phenanthrene. 

CA-CH 

C6H6-CH 

Stilbene. 

C6H.—  CH 

1           II 
C6H4-CH 

Phenanthrene. 

Again,  phenanthrene  is  formed,  together  with  anthracene, 
by  the  action  of  sodium  on  o-bromobenzyl  bromide  (p.  461), 


-rj         .,T 

6     4  = 

/CH:CH\ 


C6H4  +  H2  +  4NaBr. 


*  Stilbene,  or  diphenylethylene,  C6H5-CH  :  CH-C6H5,  may  be  prepared  by 
acting  on  benzal  chloride  (p.  349)  with  sodium, 

2C6H5-CHC12  +  4Na  =  C6H5-CH:CH-C6H5  +  4NaCl. 

It  crystallises  in  colourless  needles,  melts  at  120°,  and,  like  ethylene,  com- 
bines with  two  atoms  of  bromine,  forming  stilbene  dibromide, 

C«H5-CHBr-CHBr-C6H5  (m.p.  237°). 


470 


ANTHRACENE  AND  PHENANTHRENE. 


For  these  and  many  other  reasons,  the  constitution  of  phenan- 
threne  is  expressed  by  the  formula, 

CH  =  CH 


When  the  hydrocarbon  is  oxidised  to  phenanthraquinone, 
the  group  -CH  =  CH-  becomes  -CO — CO-,  and,  on  further 
oxidation  to  diphenic  acid,  this  group  is  converted  into  two 
carboxyl-groups, 

CO  —  CO  COOH  COOH 


Phenanthraquinone.  Diphenic  Acid. 

CH4— CO 

Phenanthraquinone,      i  i    ,  like   anthraquinone,  is 

C6H4 — CO 

formed  by  oxidising  the  hydrocarbon  with  chromic  acid.  It 
crystallises  from  alcohol  in  orange  needles,  and  melts  at  198°. 
In  chemical  properties  it  shows  little  resemblance  to  anthra- 
quinone, but  is  closely  related  to  /2-naphthaquinone  (p.  456), 
and  is,  like  the  latter,  an  ortho-diketone  (ortho-quinone) ;  it  is 
readily  reduced  by  sulphurous  acid  to  diliydroxyphenantlirene, 
C14H8(OH)2,  and  it  combines  with  sodium  bisulphite,  forming 
a  soluble  bisulphite  compound,  C14H802,  NaHS03  +  2H20 ; 
with  hydroxylamine  it  yields  a  diaxime,  C19H8(C:NOH)2. 
The  hydroxy-derivatives  of  phenanthraquinone,  unlike  those 
of  anthraquinone,  possess  no  tinctorial  properties. 

Phenanthraquinone  may  be  readily  detected  by  dissolving  a 
small  quantity  (0-1  gram)  in  glacial  acetic  acid  (20  c.c.),  adding  a  few 
drops  of  commercial  toluene,  and  then  mixing  the  well-cooled  solu- 
tion with  sulphuric  acid  (1  c.c.).  After  standing  for  a  few  minutes, 
the  bluish-green  liquid  is  poured  into  water  and  shaken  with  ether, 
when  the  ether  acquires  an  intense  reddish- violet  colouration 


ANTHRACENE  AND  PHENANTHRENE.          471 

(Laubenheimer's  reaction).  Like  the  indophenin  reaction,  this 
test  depends  on  the  formation  of  a  colouring  matter  containing 
sulphur,  produced  by  the  condensation  of  the  phenanthraquinone 
with  the  thiotolene,  C4H3S(CH3),  which  is  contained  in  the  crude 
toluene  (p.  334). 

C6H4— COOH 

Diphenic  acid,    i  ,  obtained  by  the  oxidation  of 

C6H4 — COOH 

phenantbrene  or  of  pbenanthraquinone  with  chromic  acid, 
crystallises  from  water  in  needles,  and  melts  at  229°.  When 
heated  with  acetic  anhydride  it  is  converted  into  diphenie 

anhydride,  C12H8<£°>0  (m.p.  217°). 

This  fact  is  remarkable,  because  it  shows  that  in  the  case  of 
derivatives  of  hydrocarbons  which  are  composed  of  condensed 
benzene  nuclei,  the  ortho-position  is  not  the  only  one  which  allows 
of  the  formation  of  an  anhydride.  Naphthalic  acid,  C10H6(COOH)2, 
a  derivative  of  naphthalene  in  which  the  carboxyl-groups  are  in  the 
1:1'-  or  peri-position,  also  forms  an  anhydride. 


CHAPTER    XXXII. 

PYRIDINE    AND    QUINOLTNE. 

Pyridine  and  quinoline  are  two  very  interesting  aromatic 
bases,  and  many  of  their  derivatives,  more  especially  those 
which  occur  in  nature,  are  well-known  and  important  com- 
pounds. 

Coal-tar,  though  consisting  principally  of  hydrocarbons 
and  phenols,  contains  also  small  quantities  of  pyridine,  quino- 
line, and  numerous  other  basic  substances,  such  as  aniline 
and  isoquinoline ;  all  these  bases  are  dissolved,  in  the  form 
of  sulphates,  in  the  purification  of  the  hydrocarbons,  &c., 
by  treatment  with  sulphuric  acid  (compare  p.  297),  and,  on 
afterwards  adding  excess  of  soda  to  the  dark  acid  liquor,  they 
separate  again  at  the  surface  of  the  liquid  in  the  form  of  a 
dark-brown  oil.  By  repeated  fractional  distillation  a  partial 


472  PYRIDINE  AND   QU1NOLINK 

separation  of  the  various  constituents  of  this  oil  may  be 
etfected,  and  crude  pyridine,  quinoline,  &c.,  may  be  obtained ; 
on  further  purification  by  crystallisation  of  their  salts,  or  in 
other  ways,  some  of  these  bases  may  be  prepared  in  a  state  of 
purity. 

Another  important  source  of  these  compounds  is  bone-tar 
or  bone-oil,  a  dark-brown,  unpleasant-smelling  liquid  formed 
during  the  dry  distillation  of  bones  in  the  preparation  of 
bone-black  (animal  charcoal) ;  this  oil  contains  considerable 
quantities  of  pyridine  and  quinoline,  and  their  homologues,  as 
well  as  other  bases,  and  these  compounds  may  be  extracted 
from  it  with  the  aid  of  sulphuric  acid,  and  then  separated  in 
the  manner  mentioned  above.  Bone-oil,  purified  by  distilla- 
tion, was  formerly  used  in  medicine  under  the  name  of 
Dippel's  oil. 

Pyridine  and  its  Derivatives. 

Pyridine,  C5H5N,  is  formed  during  the  destructive  distilla- 
tion of  a  great  variety  of  nitrogenous  organic  substances, 
hence  its  presence  in  coal-tar  and  in  bone-oil;  it  is  also 
formed  when  various  alkaloids  are  distilled  with  potash. 

It  may  be  obtained  synthetically  by  passing  a  mixture  of 
acetylene  and  hydrogen  cyanide  through  a  red-hot  tube,  a 
reaction  which  is  very  similar  to  that  which  occurs  in  the 
formation  of  benzene  from  acetylene  alone  (p.  301), 

2C2H2  +  HCN  -  C5H3N. 

Pyridine  is  conveniently  prepared  in  small  quantities  by 
distilling  nicotinic  acid  (p.  479),  or  other  pyridinecarboxylic 
acids,  with  lime,  just  as  benzene  may  be  prepared  from 
benzoic  and  phthalic  acids  in  a  similar  manner, 

C5H4N-COOH  =  C5H5N  +  C02 
C5H3N(COOH)2  =  C5H5N  +  2C02. 

For  commercial  purposes  it  is  usually  prepared  by  the  frac- 
tional distillation  of  the  basic  mixture,  which  is  separated 
from  bone-oil  or  coal-tar  as  already  described;  the  product 


PYRIDINE   AND   QUINOLINE.  473 

consists    of  pyridine,   together  with  small  quantities  of  its 
homologues. 

Pyridine  is  a  colourless,  mobile  liquid  of  sp.  gr.  1-0033  at 
0° ;  it  boils  at  1 1 5°,  is  miscible  with  water  in  all  proportions, 
and  possesses  a  pungent  and  very  characteristic  odour.  It  is 
an  exceedingly  stable  substance,  as  it  is  not  attacked  by 
boiling  nitric  or  chromic  acid,  and  only  with  difficulty  by  halo- 
gens ;  in  the  latter  case,  substitution  products  such  as  mono- 
bromopyridine,  C5H4BrN,  and  dibromopyridine,  C5H3Br2N, 
are  formed.  If,  however,  a  solution  of  pyridine  in  hydro- 
chloric acid  be  treated  with  bromine,  a  crystalline,  unstable, 
additive  product,  C5H5NBr2,  is  precipitated,  even  from  very 
dilute  solutions,  and  the  formation  of  this  substance  is 
sometimes  used  as  a  test  for  pyridine. 

When  treated  with  sodium  and  alcohol,  pyridine  is  readily 
reduced,  piperidine  or  hexahydropyridim  (p.  476)  being 
formed, 

C5H5N  +  6H  =  C6HUN. 

Pyridine  is  a  strong  base  ;  like  the  amines,  it  turns  red  lit- 
mus blue,  and  combines  with  acids  to  form  crystalline  salts, 
such  as  the  hydrocliloride,  C5H5N,HC1,  and  the  sulphate, 
(C5H5X)2,H2S04.  The  platinochloride,  (C^I^HgPtCle, 
crystallises  in  orange-yellow  needles,  and  is  readily  soluble 
in  water ;  when,  however,  its  solution  is  boiled,  a  very 
sparingly  soluble  yellow  salt,  (C5H5N)2PtCl4,  separates,  a 
fact  which  may  be  made  use  of  for  the  detection  of  pyridine 
even  when  only  small  quantities  of  the  base  are  available. 
Another  test  for  pyridine  (and  its  homologues)  consists 
in  heating  a  few  drops  of  the  base  in  a  test  tube 
with  methyl  iodide,  when  a  vigorous  reaction  takes  place, 
and  a  yellowish  additive  product,  pyridine  methiodide, 
C5H5N,CH3I,  is  produced  ;  if  a  piece  of  solid  potash  be 
now  added,  and  the  contents  of  the  tube  again  heated,  a 
most  pungent  and  exceedingly  disagreeable  smell  is  at  once 
noticed. 

Constitution. — Although  pyridine  is  a  powerful  base,  having 


474  PYRIDINE    AND    QUINOLINE, 

a  pungent  odour,  and  turning  red  litmus  blue,  properties 
which  suggest  some  relation  to  the  fatty  amines,  a  careful 
consideration  of  its  molecular  formula  and  chemical  behaviour 
shows  at  once  that  it  is  not  analogous  to  the  fatty  amines  in 
constitution.  It  is  not  a  primary,  nor  a  secondary  amine, 
because  it  does  not  give  the  carbylamine  reaction,  and  is  not 
acted  on  by  nitrous  acid,  and  it  cannot  possibly  be  a  tertiary 
fatty  amine,  because  no  reasonable  constitutional  formula 
based  on  this  view  could  be  constructed.  If,  moreover,  it  be 
borne  in  mind  that  pyridine  is  extremely  stable,  the 
probability  of  its  being  a  fatty  (open-chain)  compound 
at  all  seems  very  remote,  because  if  it  were,  it  would  be 
highly  unsaturated,  and  should  be  readily  oxidised  and  resolved 
into  simpler  substances.  The  grounds  for  doubting  its 
relation  to  any  fatty  compound  are,  in  fact,  much  the  same 
as  those  which  led  to  the  conclusion  that  the  constitution 
of  benzene  is  totally  different  from  that  of  dipropargyl  (p. 
304). 

Comparing  now  the  properties  of  pyridine  with  those  of 
aromatic  compounds,  a  general  analogy  is  at  once  apparent ;  in 
spite  of  its  great  stability,  pyridine  is  really  an  unsaturated 
compound,  and,  like  benzene,  naphthalene,  and  other  closed- 
chain  compounds,  it  yields  additive  products  under  certain 
conditions,  although  as  a  rule  it  gives  substitution  products. 

Considerations  such  as  these  led  to  the  conclusion,  suggested 
by  Korner  in  1869,  that  pyridine,  like  benzene,  contains  a 
closed-chain  or  nucleus,  as  represented  by  the  following 
formula, 

c 


and  this  view  has  since  been  confirmed  in  a  great  many  ways, 
notably  in  the  following  manner  :  Piperidine,  or  hexahydro- 
pyridine,  the  compound  which  is  formed  by  the  reduction  of 


PYRID1NE   AND   QUlNOLtNE. 


475 


pyridine,  and  which  is  reconverted  into  the  latter  on 
oxidation  with  sulphuric  acid  (p.  477),  has  been  prepared 
synthetically  by  a  method  (p.  478)  which  shows  it  to  have  the 
constitution  (i.);  pyridine,  therefore,  has  the  constitution  (IL), 
the  relation  between  the  two  compounds  being  the  same  as 
that  between  benzene  and  hexahydrobenzene. 


CH 


CH 


NH 
Piperidine  (I.). 


p       ^iC 

\^  ^Jc 

N 
Pyridine  (II.X 


That  the  constitution  of  pyridine  is  represented  by  this 
formula  (u.)  is  also  established  by  a  study  of  the  isomerism 
of  pyridine  derivatives,  and  by  its  relation  to  quinoline 
(p.  482) ;  it  must,  therefore,  be  regarded  as  derived  from 
benzene  by  the  substitution  of  trivalent  nitrogen  N<^  for  one 
of  the  CH<:  groups. 

The  exact  nature  of  the  union  of  the  nitrogen  and  carbon  atoms 
is  not  known,  and  as  in  the  case  of  benzene,  several  methods  of 
representation  (some  of  which  are  shown  below)  have  been  sug- 
gested ;  of  these,  the  centric  formula  is  perhaps  the  best,  for 
reasons  similar  to  those  already  mentioned  in  discussing  the  con- 
stitution of  benzene  (pp.  306,  307). 


CH 


Korner. 


CH 


CH 


Centric  Formula. 


Isomerism  of  Pyridine  Derivatives. — The  ?nora0-substitution 
products  of  pyridine,  as,  for  example,  the  methylpyridines  or 
picolines,  exist  in  three  isomeric  forms ;  this  fact  is  clearly 
in  accordance  with  the  accepted  constitutional  formula  for 
pyridine,  in  which,  for  the  sake  of  reference,  the  carbon 


476  PYRIDINE    AND   QUINOLINE. 

atoms  may  be  numbered  or  lettered  in  the  following  manner, 
the  symbols  C  and  H  being  omitted  as  usual : 


These  substitution  products,  being  formed  by  the  displace- 
ment of  any  one  of  the  five  hydrogen  atoms,  it  is  evident 
that  the  following  three  (but  not  more  than  three),  isomerides 
may  be  obtained : 


:  '          I' 


The  positions  aa1  (or  1,  5)  are  identical,  and  so  also  are  the 
positions  /3fil  (or  2,  4),  but  the  position  y  (or  3)  is  different 
from  any  of  the  others. 

The  ^^'-substitution  products  exist  theoretically  in  six 
isomeric  forms,  the  positions  of  the  substituents  in  the 
several  isomerides  being  as  follows  : 

1:2,  1:3,  1:4,  1:5,  2:3,  2:4. 

All  other  positions  are  identical  with  one  of  these ;  4 : 5,  for 
example,  is  the  same  as  1:2,  and  3:4  is  identical  with  2:3. 

As  regards  the  isomerism  of  its  derivatives,  pyridine  may 
be  conveniently  compared  with  a  mono-substitution  product 
of  benzene — aniline,  for  example — the  effect  of  substituting  a 
nitrogen  atom  for  one  of  the  CH<J  groups  in  benzene  being 
the  same,  in  this  respect,  as  that  of  displacing  one  of  the 
hydrogen  atoms  by  some  substituent. 

Deri  Datives  of  Pyridine. — Piperidine,  or  hexahydropyridine, 
C5H10NH,  is  formed,  as  already  stated,  when  pyridine  is 
reduced  with  sodium  and  alcohol ;  it  is  usually  prepared 
from  pepper,  which  contains  the  alkaloid  piperine  (p.  490),  a 


PYRIDINE    AND    QUINOLINE.  477 

substance  which  is  decomposed  by  boiling  alkalies  yielding 
piperidine  and  piperic  acid. 

Powdered  pepper  is  extracted  with  alcohol,  the  filtered  solution 
evaporated,  and  the  residue  distilled  with  potash  ;  after  neutralis- 
ing with  hydrochloric  acid,  the  distillate  is  evaporated  to  dryness, 
and  the  residue  extracted  with  hot  alcohol  to  separate  the  piperi- 
dine hydrochloride  from  the  ammonium  chloride  which  is  always 
present.  The  filtered  alcoholic  solution  is  then  evaporated,  the 
residue  distilled  with  solid  potash,  and  the  crude  piperidine  purified 
by  fractional  distillation  over  potash. 

Piperidine  is  a  colourless  liquid,  boiling  at  106°,  and  is 
miscible  with  water  in  all  proportions,  heat  being  developed ; 
it  has  a  very  penetrating  odour,  recalling  that  of  pepper. 
Like  pyridine,  it  is  a  very  strong  base,  turns  red  litmus  blue, 
and  combines  with  acids  forming  crystalline  salts ;  when 
heated  with  concentrated  sulphuric  acid  at  300°,  it  loses  six 
atoms  of  hydrogen,  and  is  converted  into  pyridine,  part  of 
the  sulphuric  acid  being  reduced  to  sulphur  dioxide. 

Piperidine  behaves  like  a  secondary  amine  towards  nitrous 
acid,  and  yields  nitroso-piperidine,  C5H10N-XO,  an  oil,  boiling 
at  218°;  like  secondary  amines,  moreover,  it  interacts  with 
methyl  iodide,  giving  methylpiperidine,  C5H10N-CH3;  it  is, 
therefore,  a  secondary  base  (compare  p.  483). 

The  important  synthesis  of  piperidine,  which  has  already 
been  referred  to  as  establishing  the  constitution  of  the  base, 
and  also  that  of  pyridine,  was  accomplished  by  Ladenburg 
in  the  following  way :  Trimethylene  bromide*  is  heated 
with  potassium  cyanide  in  alcoholic  solution,  and  thus  con- 
verted into  trimethylene  cyanide, 

Br.CH2.CH2.CH2.Br  +  2KCN  =  CN.CH2-CH2.CH2.CN  +  2KBr, 

a  substance   which,   on  reduction  with   sodium  and  alcohol, 

*  Trimethylene  bromide,  C3H6Br2,  is  prepared  by  treating  allyl  bromide 
(part  i.  p.  255)  with  concentrated  hydrobromic  acid, 

CH2Br-CH:CH2  +  HBr  =  CH2Br-CH2-CH2Br; 

it  is  a  heavy,  colourless  oil,  and  boils  at  164°. 


478  PYRIDINE    AND    QUINOLINE. 

yields  pentametliylene  diamine,  just  as  methyl  cyanide  under 
similar  conditions  yields  ethylamine, 

CN-CH2.CH2.CH2.CN  +  8H  =  NH2.CH2.CH2-CH2.CH2.CH2.NH2  ; 
during  this  reduction  process,  some  of  the  pentametliylene 
diamine  is  decomposed  into  piperidine  and  ammonia,  and  the 
same  change  occurs,  but  much  more  completely,  when  the 
hydrochloride  of  the  diamine  is  distilled, 

CH  <C 
M 


Homologues  of  Pyridine.  —  The  alkyl-derivatives  of  pyridine 
occur  in  coal-tar  and  bone-oil,  and  are,  therefore,  present  in 
the  crude  pyridine  obtained  from  the  mixture  of  bases  in 
the  manner  referred  to  above  ;  they  can  only  be  isolated  by 
repeated  fractional  distillation  and  subsequent  crystallisation 
of  their  salts.  The  three  (a,  /?,  y)  isomeric  metliylpyridims 
or  picolines,  C5H4N-CH3,  the  six  isomeric  dimethylpyridines 
or  lutidines,  C5H3N(CH3)2,  and  the  trimetliylpyridines  or 
collidines,  C5H2N(CH3)3,  resemble  the  parent  base  in  most 
ordinary  properties,  but,  unlike  the  latter,  they  undergo  oxida- 
tion more  or  less  readily  on  treatment  with  nitric  acid  or 
potassium  permanganate,  and  are  converted  into  pyridine- 
carboxylic  acids,  just  as  the  homologues  of  benzene  yield 
benzenecarboxylic  acids,  the  alkyl-groups  or  side-chains  being 
oxidised  to  carboxyl-groups, 

C5H4JST.CH3  +  30  =  C5H4N.COOH  +  H20 
C5H3N(CH3)2  +  60  =  C6HSN(COOH)2V2H20. 

This  behaviour  is  of  great  use  in  determining  the  positions  of 
the  alkyl-groups  in  these  homologues  of  pyridine,  because  the 
carboxylic  acids  into  which  they  are  converted  are  easily 
isolated,  and  are  readily  identified  by  their  melting-points 
and  other  properties. 

The  .pyridinecarboxylic  acids  are  perhaps,  as  a  class,  the 
most  important  derivatives  of  pyridine,  chiefly  because  they 
are  obtained  as  decomposition  products  on  oxidising  many  of 
the  alkaloids. 


PYRIDINE    AND    QUIXOLINE.  479 

The  three  (a,  /?,  y)  monocarboxylic  acids  may  be  prepared 
by  oxidising  the  corresponding  picolines  or  methylpyridines 
(see  above)  with  potassium  permanganate.  The  a-carboxylic 
acid  is  usually  known  as  picolinic  acid,  because  it  was  first 
prepared  from  a-picoline  (a-methylpyridine),  whereas  the 
/^-compound  is  called  nicotinic  acid,  because  it  was  first 
obtained  by  the  oxidation  of  nicotine  (p.  489);  the  third 
isomeride — namely,  the  y-carboxylic  acid,  is  called  isonicotinic 
acid,  and  is  the  oxidation  product  of  y-picoline. 

COOH 

OCOOH 

N  N  N 

Picolinic  Acid,  or  Nicotinic  Acid,  or  Isonicotinic  Acid,  or 

Pyridine-*-carboxylic  Acid  Pyridine-/3-carboxylic  Acid   Pyridine-y-carboxylic  Acid 
(in. p.  136°).  (m.p.  229°).  (sublimes  without  melting). 

These  monocarboxylic  acids  are  all  crystalline  and  soluble 
in  water ;  they  have  both  basic  and  acid  properties,  and  form 
salts  with  mineral  acids  as  well  as  with  bases,  a  behaviour 
which  is  similar  to  that  of  glycine  (part  i.  p.  292). 

The  a-carboxylic  acid,  and  all  other  pyridinecarboxylic 
acids  which  contain  a  carboxyl-group  in  the  a-position  (but 
only  such),  give  a  red,  or  yellowish-red  colouration  with  ferrous 
sulphate,  a  reaction  which  is  of  great  value  in  determining 
the  positions  of  the  carboxyl-groups  in  such  compounds. 

A  carboxyl-group  in  the  a-position,  moreover,  is  usually 
very  readily  eliminated  on  heating;  picolinic  acid,  for 
example,  is  much  more  readily  converted  into  pyridine  than 
nicotinic  or  isonicotinic  acid. 

Quinolinic  acid,  C5H3X(COOH).7  (pyridine-a/3-dicarboxylic 
acid), 


N 

a   compound    produced    by  the   oxidation  of  quinoline  with 


480  PYRIDINE    AND    QUINOLINE. 

potassium  permanganate,  is  the  most  important  of  the  six  iso- 
meric  dicarboxylic  acids.  It  crystallises  in  colourless  prisms, 
is  only  sparingly  soluble  in  water,  and  gives,  with  ferrous  sul- 
phate, an  orange 'Colouration,  one  of  the  carboxyl-groups  being 
in  the  a-position.  When  heated  at  190°  it  decomposes  into 
carbon  dioxide  and  nicotinic  acid,  a  fact  which  shows  that 
the  second  carboxyl-group  is  in  the /3-position.  On  distilla- 
tion with  lime,  quinolinic  acid,  like  all  pyridinecarboxylic 
acids,  is  converted  into  pyridine. 

In  its  behaviour  when  heated  alone,  quinolinic  acid  differs 
in  a  marked  manner  from  phthalic  acid — the  corresponding 
benzenedicarboxylic  acid — as  the  latter  is  converted  into 
its  anhydride  (p.  426) ;  nevertheless,  when  heated  with 
acetic  anhydride,  quinolinic  acid  gives  an  anhydride, 

a  colourless,  crystalline  substance,  melting 

at  134°.  This  fact  shows  that  the  carboxyl-groups  are  united 
with  carbon  atoms,  which  are  themselves  directly  united  (as 
in  the  case  of  phthalic  acid),  and  is  further  evidence  in 
support  of  the  constitutional  formula  given  above. 

Quinoline. 

Quinoline,  C9H7N,  occurs,  together  with  isoquinoline,  in 
that  fraction  of  coal-tar  and  bone-oil  bases  (p.  472)  which  is 
collected  between  236  and  243°,  but  as  it  is  difficult  to  obtain 
the  pure  substance  from  this  mixture,  quinoline  is  usually  pre- 
pared synthetically,  by  a  method  devised  by  Skraup.  For 
this  purpose  a  mixture  of  aniline  and  glycerol  is  heated 
with  a  dehydrating  agent  (sulphuric  acid)  and  an  oxidising 
agent,  such  as  nitrobenzene.* 

A  mixture  of  aniline  (38  parts),  concentrated  sulphuric  acid  (100 
parts),  nitrobenzene  (24  parts),  and  glycerol  (120  parts),  is  cautiously 
heated  (with  reflux  apparatus)  on  a  sand-bath,  and  after  the  violent 
reaction  which  soon  sets  in  has  subsided,  the  mixture  is  kept  boiling 

*  Nitrobenzene  is  often  employed  as  a   mild   oxidising    agent,   as,   in 
presence  of  an  oxidisable  substance,  it  is  reduced  to  aniline, 
C6H5-N02  +  2H  =  C6H5-NH2  +  2O. 


PYRIDINE    AND    QUINOLINE.  481 

for  about  four  hours.  It  is  then  cooled,  diluted  with  water,  and  the 
unchanged  nitrobenzene  separated  by  distillation  in  steam  ;  soda 
is  then  added  in  excess  to  liberate  the  quinoline  from  its  sulphate, 
and  the  mixture  is  again  steam-distilled.  The  quinoline  in  the 
receiver  is  finally  separated  with  the  aid  of  a  funnel,  dried  over 
solid  potash,  and  purified  by  fractional  distillation. 

Quinoline  is  a  colourless,  highly  refractive  oil,  of  sp.  gr. 
1-095  at  20°,  and  boils  at  239°.  It  has  a  peculiar  charac- 
teristic smell,  and  is  sparingly  soluble  in  water,  but  it  dissolves 
freely  in  dilute  acids,  forming  crystalline  salts,  such  as  the 
hydrochloride,  C9H7N,HC1,  the  sulphate,  (C9H7N)2,H2S04,  &c. 
It  also  forms  double  salts,  of  which  the  platinochloride, 
(C9H7N)2,H2PtCl6  +  2H20,  and  the  bichromate, 

(C9H7N)2,H20207, 

may  be  mentioned ;  the  latter,  prepared  by  adding  potassium 
bichromate  to  a  solution  of  quinoline  hydrochloride,  crystal- 
lises from  water,  in  which  it  is  only  sparingly  soluble,  in 
glistening  yellow  needles,  melting  at  164-167°. 

Quinoline  is  a  tertiary  base  (compare  p.  484),  and  com- 
bines, with  methyl  iodide,  to  form  the  additive  product, 
quinoline  methiodide,  C9H7N,CH3I. 

Constitution. — As  the  relation  between  pyridine,  C5H5N, 
and  quinoline,  C9H7N,  on  the  one  hand,  is  much  the  same  as 
that  between  benzene,  C6H6,  and  naphthalene,  C10H8,  on  the 
other,  both  as  regards  chemical  behaviour  and  molecular 
composition  (the  difference  being  C4H2  in  both  cases),  it 
might  be  assumed  that  quinoline  is  derived  from  pyridine, 
just  as  naphthalene  is  derived  from  benzene  ;  consequently 
the  constitution  of  quinoline  might  be  expressed  by  one  of  the 
following  formulas : 

CH  CH  CH  CH 

^^\CH 


CH  N  CH  CH 

I.  II. 

Now,   quinoline  differs   from  pyridine,  just   as   naphthalene 

2E 


PYRIDINE    AND    QUINOLINE. 

differs  from  benzene,  in  being  much  more  readily  oxidised, 
and  when  heated  with  potassium  permanganate  it  yields 
quinolinic  acid,  C5H3N(COOH)2,  a  derivative  of  pyridine  (p. 
479)  ;  this  fact  proves  that  quinoline  contains  a  pyridine 
nucleus;  but  it  also  contains  a  benzene  nucleus,  as  is  shown 
by  its  formation  from  aniline  by  Skraup's  method.  Its  con- 
stitution must,  therefore,  be  expressed  by  one  of  the  above 
formulae,  as  these  facts  admit  of  no  other  interpretation.  As, 
moreover,  the  carboxyl-groups  in  quinolinic  acid  are  in  the 
a:/3-position  (compare  p.  480),  formula  u.  is  inadmissible,  a 
conclusion  which  is  obviously  necessary  to  explain  the  forma- 
tion of  quinoline  from  aniline.  For  these  and  other  reasons, 
the  constitution  of  quinoline  is  represented  by  formula  I.  (the 
other  expressing  that  of  isoquinoline). 

The  formation  of  quinoline  from  aniline  and  glycerol  may 
be  explained  as  follows :  The  glycerol  and  sulphuric  acid 
first  interact,  yielding  acrolein  (part  i.  pp.  249,  256),  which 
then  condenses  with  aniline  (as  do  all  aldehydes),  forming 
acrylaniline, 

C6H5-NH2  +  CHO-CH:CH2  =  C6H5.N:CH-CH:CH2  +  H20; 

this  substance,  under  the  oxidising  action  of  the  nitro- 
benzene, loses  two  atoms  of  hydrogen,  and  is  converted  into 
quinoline, 

CH  CH2  CH  CH 


+  O  =  +   H20. 

CH1 


Many  derivatives  of  quinoline  may  be  obtained  by  Skraup's 
method,  using  derivatives  of  aniline  instead  of  the  base 
itself;  when,  for  example,  one  of  the  three  toluidines  (p.  364) 
is  employed,  a  metliylquinoline  is  formed,  the  position  of  the 
methyl- group- — which  is,  of  course,  united  with  the  benzene 
and  not  with  the  pyridine  nucleus — depending  on  which  of  the 
toluidines  is  taken. 


PYRIDINE   AND   QUINOL1XE. 


483 


•Isoquinoline,  C9H7N,  occiirs  iu  coal-tar  quiiioline,  and  may  be 
isolated  by  converting  the  fraction  of  the  mixed  bases,  boiling  at 
236-243°,  into  the  acid  sulphates,  C9H7N,H2S04,  and  recrystallising 
these  salts  from  alcohol  (88  per  cent.)  until  the  crystals  melt  at  205°. 
The  sulphate  of  isoquinoline  thus  obtained  is  decomposed  by  potash, 
and  the  base  purified  by  distillation,  Isoquinoline  is  very  like 
quinoline  in  chemical  properties,  but  it  is  solid,  and  melts  at  22° ; 
its  boiling-point,  241°,  is  also  slightly  higher  than  that  of  quinoline 
(239°). 

The  constitution  of  isoquinoline  is  very  clearly  proved  by  its 
behaviour  on  oxidation  with  permanganate,  when  it  yields  both 
phthalic  acid  and  cinchomeronic  acid,  C5H3N(COOH)2,  or  pyridine- 
/fy-dicarboxylic  acid ;  oxidation  takes  place,  therefore,  in  two 
directions,  in  the  one  case  the  pyridine  (Py),  in  the  other  the 
benzene  (B),  nucleus  being  broken  up. 


CH 


Isoquinoline. 


CH  CH 

If          ^C-COOH  COOH-Cf"          \CH 
B 

JC-COOH  COOH-< 
CH  CH 

Phthalic  Acid.  Cinchomeronic  Acid. 


Secondary  and  Tertiary  Aromatic  Bases.  —  Compounds  such 
as  pyridine,  piperidine,  and  quinoline,  which  owe  their  basic 
character  to  the  presence  of  nitrogen  forming  part  of  a  closed- 
chain  or  nucleus,  are  classed  as  secondary  or  tertiary  bases, 
according  as  the  nitrogen  atom  is  combined  with  hydrogen, 
as  well  as  with  carbon,  or  only  with  the  latter. 

The  secondary  bases,  such  as  piperidine,  which  contain  an 
^>NH-group,  show  in  some  respects  the  behaviour  of 
secondary  amines.  When  treated  with  nitrous  acid  they  yield 
nitroso-derivatives  (which  give  Liebermann's  reaction), 


>XH  +  HO-NO  =  >N-NO  +  H20, 

and  when  warmed  with  an  alkyl  halogen  compound,  such  as 
methyl  iodide,  they  are  converted  into  alkyl-derivatives  by 
the  substitution  of  an  alkyl-group  for  the  hydrogen  atom  of 
the  >NH-group, 

CH3I  =  >N-CH3,HI, 


484  PTRIDINE   AND    QUINOLINE. 

just  as  diethylamine,  for  example,  interacts  with  ethyl  iodide, 
giving  triethylamine, 

(C2H5)2NH  +  C2H5I  =  (C2H5)2N.C2H5,HI. 

These  alkyl-derivatives  of  the  secondary  bases  are  them- 
selves tertiary  bases,  and  have  the  property  of  forming 
additive  products  with  alkyl  halogen  compounds,  giving 
salts  corresponding  with  the  quaternary  ammonium  salts 
(part  i.  pp.  204,  205), 

>N.CH3  +  CH3T  =  :>N.CH3,CH3I,  or  >N(CHS)2I. 
The  hydrogen  atom  of  the>NH-group  in  secondary  bases 
is  also  displaceable  by  the  acetyl-group  and  by  other  acid 
radicles. 

The  tertiary  bases,  such  as  pyridine  and  quinoline,  in  which 
the  nitrogen  atom  is  not  directly  united  with  hydrogen, 
behave  in  many  respects  like  the  tertiary  amines  ;  they  do  not 
yield  nitroso-  nor  acetyl-derivatives,  but  when  treated  with  an 
alkyl  halogen  compound  they  yield  additive  compounds,  cor- 
responding with  the  quaternary  ammonium  salts,  without  the 
formation  of  any  intermediate  product, 

CH3I  =  >N,CH3I,  or 


These  differences  in  behaviour  make  it  an  easy  matter  to 
distinguish  between  secondary  and  tertiary  aromatic  bases  of 
this  class. 


CHAPTER    XXXIII. 

ALKALOIDS. 

The  alkaloids,  like  the  carbohydrates  (part  i.  p.  259),  do 
not  form  a  well-defined  group,  this  term  being  applied  to 
nearly  all  basic  nitrogenous  substances  which  occur  in  plants, 
irrespective  of  any  similarity  in  properties  or  constitution. 

Most  alkaloids  are  composed  of  carbon,  hydrogen,  oxygen, 
and  nitrogen,  and  are  crystalline  and  non-volatile,  but  a  few, 


ALKALOIDS.  485 

notably  coniine  and  nicotine,  are  composed  of  carbon,  hydro- 
gen, and  nitrogen  only,  and  are  volatile  liquids ;  with  the 
exception  of  these  liquid  compounds,  which  are  readily 
soluble,  the  alkaloids  are  usually  sparingly  soluble  in  water, 
but  dissolve  much  more  readily  in  alcohol,  chloroform,  ether, 
and  other  organic  solvents ;  they  are  all  soluble  in  acids,  with 
which  they  usually  form  well-defined,  crystalline  salts.  Many 
alkaloids  have  a  very  bitter  taste,  and  are  excessively  poison- 
ous ;  many,  moreover,  are  extensively  used  in  medicine,  and 
their  value  in  this  respect  can  hardly  be  overrated. 

Generally  speaking,  the  alkaloids  are  tertiary  aromatic  bases, 
but,  with  few  exceptions,  their  constitutions  have  not  been 
established,  owing  partly  to  their  complexity,  partly  to  the 
difficulties  which  are  experienced  in  resolving  them  into 
simpler  compounds  which  throw  any  light  on  the  structure 
of  their  molecules.  Nevertheless,  work  has  been  done  in  this 
direction,  and  it  is  known  that  many  alkaloids  are  derivatives 
of  pyridine,  or  of  quinoline,  because  they  yield  these  bases,  or 
their  derivatives,  when  strongly  heated  with  potash,  and,  on 
oxidation,  usually  with  potassium  permanganate,  they  give 
carboxylic  acids  of  pyridine  and  quinoline. 

It  is  a  remarkable  fact  that  by  far  the  greater  number  of 
alkaloids  contain  one  or  two,  sometimes  three  or  more, 
methoxy-groups  (-0-CH3),  united  with  a  benzene  nucleus 
(as  in  anisole,  C6H5-0-CH3,  p.  392),  and  the  determination 
of  the  number  of  such  groups  in  the  molecule  is  of  the 
greatest  importance  in  establishing  the  constitution  of  an 
alkaloid,  because  in  this  way  some  of  the  carbon  and  hydro- 
gen atoms  are  at  once  disposed  of.  The  method  employed 
for  this  purpose  depends  on  the  fact  that  all  substances  con- 
taining inethoxy-groups  are  decomposed  by  hydriodic  acid, 
yielding  methyl  iodide  and  a  hydroxy-compound  (compare 
anisole)  in  accordance  with  the  general  equation, 

n(-0-CH8)  +  nRl  =  w(-OH)  +  wCH3I; 
by  estimating  the  amount  of  methyl  iodide  obtained  from  a 


(  ALKALOIDS. 

known  weight  of  a  given  compound,  it  is  easy,  therefore,  to 
determine  the  number  of  methoxy-groups  in  the  molecule. 

This  method  was  first  applied  by  Zeisel,  and  is  of  general 
application,  as  it  affords  a  means  of  accurately  determin- 
ing the  number  of  methoxy-groups,  not  only  in  alkaloids, 
but  in  any  other  substances  in  which  they  occur ;  it  is  carried 
out  as  follows : 

A  distilling  flask  of  about  35  c.c.  capacity  (A,  fig.  20),  with  the 
side-tube  bent  as  shown,  and  suspended  in  a  beaker  of  glycerol, 
is  fixed  to  the  condenser  (B)  by  means  of  a  cork,  and  connected 
with  an  apparatus  for  generating  carbon  dioxide. 

The  condenser,  through  which  water  at  50°  circulates  from  the 
bottle  (C),  is  attached  to  the  'potash  bulbs,'  which  contain  water 
and  about  0-5  gram  of  amorphous  phosphorus ;  the  bulbs  are  sus- 
pended in  a  beaker  of  water  kept  at  60°,  and  connected,  as  shown, 
with  two  flasks  (D,  E),  containing  respectively  50  c.c.  and  25  c.c.  of 
an  alcoholic  solution  of  silver  nitrate  (prepared  by  adding  100  c.c.  of 
absolute  alcohol  to  a  solution  of  5  grams  of  silver  nitrate  in  12  c.c. 
of  water); 

In  carrying  out  the  estimation,  about  0-3  gram  of  the  substance 
under  examination  is  placed  in  the  flask  A,  together  with  10  c.c.  of 
fuming  hydriodic  acid,  and  the  temperature  of  the  glycerol  bath 
is  gradually  raised,  until  the  acid  just  boils,  carbon  dioxide,  at  the 
rate  of  about  3  bubbles  in  2  seconds,  being  passed  all  the  time. 

The  methyl  iodide  thus  formed  is  carried  forward  through  the 
Condenser  into  the  'potash  bulbs,'  where  it  is  freed  from  hydriodic 
acid  and  from  small  quantities  of  iodine,  which  it  always  contains  ; 
it  then  passes  into  the  alcoholic  silver  nitrate  solution,  and  is  de- 
composed with  separation  of  silver  iodide.  The  operation,  which 
occupies  about  two  hours,  is  at  an  end  when  the  precipitate  in  the 
flask  settles,  and  leaves  a  clear,  supernatant  liquid. 

The  contents  of  flask  E  are  poured  into  5  vols.  of  water  and 
gently  warmed ;  if,  as  is  usually  the  case,  no  precipitation  takes 
place  after  five  minutes,  the  solution  is  neglected  ;  if,  however,  a 
precipitate  forms,  it  must  be  collected  and  added  to  that  contained 
in  flask  D.  The  alcoholic  liquid  in  flask  D  is  decanted  from  the 
precipitate,  mixed  with  water  (300  c.c.)  and  a  few  drops  of  nitric 
acid,  and  heated  to  boiling  until  free  from  .alcohol ;  any  pre- 
cipitate is  then  added  to  the  main  quantity,  the  whole  digested 
for  a  few  minutes  with  dilute  nitric  acid,  collected  on  a  filter,  dried, 
and  weighed. 


488  ALKALOIDS. 

The  extraction  of  alkaloids  from  plants,  and  their  subsequent 
purification,  are  frequently  matters  of  considerable  difficulty, 
partly  because  in  many  cases  a  number  of  alkaloids  occur 
together,  partly  because  of  the  neutral  and  acid  substances, 
such  as  the  glucosides,*  sugars,  tannic  acid,  malic  acid,  &c., 
which  are  often  present  in  large  quantities.  Generally  speak- 
ing, they  may  be  extracted  by  treating  the  macerated  plant  or 
vegetable  product  with  dilute  acids,  which  dissolve  out  the 
alkaloids  in  the  form  of  salts ;  the  filtered  solution  may  then 
be  treated  with  soda  to  liberate  the  alkaloids,  which,  being 
sparingly  soluble,  are  usually  precipitated,  and  may  be  separ- 
ated by  filtration ;  if  not,  the  alkaline  solution  is  extracted 
with  ether,  chloroform,  &c.  The  products  are  finally  purified 
by  recrystallisation,  or  in  some  other  manner. 

Most  alkaloids  give  insoluble  precipitates  with  a  solution  of 
tannic,  picric,  phosphomolybdic,  or  phosphotungstic  acid, 
and  with  a  solution  of  mercuric  iodide  in  potassium  iodide,t 
&c. ;  these  reagents,  therefore,  are  often  used  for  their  detec- 
tion and  isolation. 

Only  the  more  important  alkaloids  are  described  in  the 
following  pages. 

Alkaloids  derived  from  Pyridine. 

Coniine,  CgHjyN,  one  of  the  simplest  known  alkaloids,  is 
contained  in  the  seeds  of  the  spotted  hemlock  (Conium  macula- 
tum),  from  which  it  may  be  prepared  by  distillation  with  soda. 

It  is  a  colourless  oil,  boiling  at  167°,  and  is  readily  soluble 
in  water ;  it  has  a  most  penetrating  odour,  and  turns  brown 

*  The  term  glucos\de  is  applied  to  all  those  vegetable  products  which, 
on  treatment  with  acids  or  alkalies,  yield  a  sugar,  or  some  closely  allied 
carbohydrate  and  one  or  more  other  substances  (which  are  frequently 
phenols  or  aromatic  aldehydes)  as  decomposition  products  (compare  amyg- 
dalin,  p.  405  ;  salicin,  p.  404 ;  ruberythric  acid,  p.  465,  &c.). 

•f"  For  the  preparation  of  these  solutions  larger  works  must  be  consulted. 
In  cases  of  alkaloid  poisoning  it  is  usual,  after  using  the  stomach-pump,  to 
wash  out  the  stomach  with  dilute  tannic  acid,  or  to  administer  strong  tea 
(which  contains  tannin),  in  order  to  render  the  alkaloids  insoluble,  and, 
therefore,  harmless. 


ALKALOIDS,  489 

on  exposure  to  air.  Con  line  is  a  strong  base,  and  com- 
bines with  acids  to  form  salts,  such  as  the  hydrochloride, 
C8H17N,HC1,  which  are  readily  soluble  in  water ;  both  the 
base  arid  its  salts  are  exceedingly  poisonous,  a  few  drops  of 
the  pure  substance  causing  death  in  a  short  time  by  paralysing 
the  muscles  of  respiration. 

Ladenburg  has  shown  that  coniine  is  dextrorotatory  a-propyl- 
piperidine, 

CH2 


NH 

and   has   succeeded    in    preparing  it    synthetically,  the  first 
instance  of  the  synthesis  of  an  optically  active  alkaloid. 

a-Propylpiperidine  contains  an  asymmetric  carbon  atom  (shown 
in  heavy  type — compare  p.  533),  and,  therefore,  like  lactic  acid,  it 
exists  in  three  modifications,  all  of  which  have  been  synthetically 
prepared ;  the  inactive  modification  may  be  separated  into  the 
two  optically  active  compounds  by  crystallisation  of  its  tartrate 
(compare  p.  544). 

x^Nicotine,  C10H14N2,  is  present  in  the  leaves  of  the  tobacco 
y  plant  (Nicotiana  tabacum),  combined  with  malic  or  citric  acid. 

Tobacco  leaves  are  extracted  with  boiling  water,  the  extract 
concentrated,  mixed  with  milk  of  lime,  and  distilled  ;  the  distillate 
is  acidified  with  oxalic  acid,  evaporated  to  a  small  bulk,  decomposed 
with  potash,  and  the  free  nicotine  extracted  with  ether.  The 
ethereal  solution,  on  evaporation,  deposits  the  crude  alkaloid,  which 
is  purified  by  distillation  in  a  stream  of  hydrogen. 

Nicotine  is  a  colourless  oil,  which  boils  at  24 1°,  possesses 
a  very  pungent  odour,  and  rapidly  turns  brown  on  exposure 
to  air;  it  is  readily  soluble  in  water  and  alcohol.  It  is  a 
strong  di-acid  base,  and  forms  crystalline  salts,  such  as  the 
hydrochloride,  C10HUN2,2HC1;  it  combines  directly  with  two 
molecules  of  methyl  iodide,  yielding  nicotine  dimethiodide, 
C10H14N2,2CH3I,  a  fact  which  shows  that  it  is  a  di-tertiary 
base  (p.  484).  When  oxidised  with  chromic  acid,  it  yields 


490  ALKALOIDS. 

nicotinic  acid  (pyridine-/?-carboxylic  acid,  p.  479) ;  it  is, 
therefore,  a  pyridine-derivative,  but  its  constitution  has  not 
yet  been  determined. 

Nicotine  is  exceedingly  poisonous,  two  or  three  drops  taken 
into  the  stomach  being  sufficient  to  cause  death  in  a  few 
minutes.  It  shows  no  very  characteristic  reactions,  but  its 
presence  may  be  detected  by  its  extremely  pungent  odour 
(which  recalls  that  of  a  foul  tobacco  pipe). 

Piperine,  C17H19N03,  occurs  to  the  extent  of  about  8-9  per 
cent,  in  pepper,  especially  in  black  pepper  (Piper  nigrum), 
from  which  it  is  easily  extracted. 

The  pepper  is  powdered  and  warmed  with  milk  of  lime  for  15 
minutes ;  the  mixture  is  then  evaporated  to  dry  ness  on  a  water- 
bath,  extracted  with  ether,  the  ethereal  solution  evaporated,  and 
the  residual  crude  pipeline  purified  by  recrystallisation  from  alcohol. 

It  crystallises  in  prisms,  melts  at  128°,  and  is  almost 
insoluble  in  water ;  it  is  only  a  very  weak  base,  and  when 
heated  with  alcoholic  potash,  it  is  decomposed  into  piperidine 
(p.  476)  and  piperic  acid, 

C^pNO,  +  H20  =  C5HUN  +  C12H1004. 

Piperidine.         Piperic  Acid. 

Atropine,  or  dafcurine,  C17H2SN03,  does  not  occur  in  nature, 
although  it  is  prepared  from  the  deadly  nightshade  (Atropa 
belladonna).  This  plant  contains  two  isomeric  and  closely 
related  alkaloids  hyoscyamine  and  liyoscine,  and  the  former 
readily  undergoes  intramolecular  change  into  atropine  on 
treatment  with  bases. 

The  plant  is  pressed,  the  juice  mixed  with  potash,  and  extracted 
with  chloroform  (1  litre  of  juice  requires  4  grams  of  potash  and 
30  grams  of  chloroform);  the  chloroform  is  then  evaporated,  the 
atropine  extracted  from  the  residue  with  dilute  sulphuric  acid,  the 
solution  treated  with  potassium  carbonate,  and  the  precipitated 
alkaloid  recrystallised  from  alcohol. 

It  crystallises  from  dilute  alcohol  in  glistening  prisms,  and 
melts  at  115°;  it  is  readily  soluble  in  alcohol,  ether,  and 
chloroform,  but  almost  insoluble  in  water.  When  boiled 


ALKALOIDS.  491 

with  baryta  water  it  is  readily  hydrolysed,  yielding  tropic  acid 
and  a  base  called  tropine,  which  is  a  derivative  of  pyridine, 


C17H23N03  +  H20  =  CA'CtK  +  C8H15NO. 

Tropic  Acid.  Tropine. 

Atropine  is  a  strong  base,  and  forms  well-characterised  salts,  of 
which  the  sulphate,  (Cl7H23N03)2,H2S04,  is  readily  soluble, 
and,  therefore,  most  commonly  used  in  medicine  ;  both  the 
base  and  its  salts  are  excessively  poisonous,  0-05  —  0-2  gram 
causing  death.  Atropine  sulphate  is  largely  used  in  ophthalmic 
surgery,  owing  to  the  remarkable  property  which  it  possesses 
of  dilating  the  pupil  when  its  solution  is  placed  on  the  eye. 

Test  for  Atropine.  —  If  a  trace  of  atropine  be  moistened 
with  fuming  nitric  acid,  and  evaporated  to  dryness  on  a  water- 
bath,  it  yields  a  yellow  residue,  which,  on  the  addition  of 
alcoholic  potash,  gives  an  intense  violet  solution,  the  colour 
gradually  changing  to  red. 

Cocaine,  C17H21N04,  and  several  other  alkaloids  of  less 
importance,  are  contained  in  coca  leaves  (Enjthroxylon  coca). 

The  coca  leaves  are  extracted  with  hot  water  (80°),  the  solution 
mixed  with  lead  acetate  (in  order  to  precipitate  tannin,  &c.), 
filtered,  and  the  lead  in  the  filtrate  precipitated  with  sodium  sul- 
phate; the  solution  is  then  rendered  alkaline  with  soda,  the  cocaine 
extracted  with  ether,  and  purified  by  recrystallisation  from  alcohol. 

Cocaine  crystallises  in  colourless  prisms,  melts  at  98°,  and 
is  sparingly  soluble  in  water;  it  forms  welUcharacterised 
salts,  of  which  the  hydrochloride,  C17H21N04,HC1,  is  most 
largely  used  in  medicine.  Cocaine  is  a  very  valuable  local 
anaesthetic,  and  is  used  in  minor  surgical  operations,  as  its 
local  application  takes  away  all  sensation  of  pain  ;  it  is, 
however,  poisonous,  one  grain  injected  subcutaneously  having 
been  attended  with  fatal  results. 

When  heated  with  acids  or  alkalies,  cocaine  is  readily 
hydrolysed  with  formation  of  benzoic  acid,  methyl  alcohol,  and 
ecgonine  (a  derivative  of  tetrahydropyridine), 

C17H21N04  +  2H20  =  C6H5.COOH  +  CH3-OH  +  C9H15N03. 


492  ALKALOIDS. 

Alkaloids  derived  from  Quinoline. 

Quinine,  C00H24N202,  cinchonine  (see  below),  and  several 
other  allied  alkaloids,  occur  in  all  varieties  of  cinchona-bark, 
some  of  which  contain  as  much  as  3  per  cent,  of  quinine. 
The  alkaloids  are  contained  in  the  bark,  combined  with  tamiic 
and  quinic  acids.* 

The  powdered  bark  is  extracted  with  dilute  sulphuric  acid,  and 
the  solution  of  the  sulphates  precipitated  with  soda.  The  crude 
mixture  of  alkaloids  thus  obtained  is  dissolved  in  alcohol,  the 
solution  neutralised  with  sulphuric  acid,  and  the  sulphates,  which 
are  deposited,  repeatedly  recrystallised  from  water.  Quinine 
sulphate  is  the  least  soluble,  and  separates  out  first,  the  sulphates 
of  cinchonine  and  the  other  alkaloids  remaining  in  solution  ;  from 
the  pure  sulphate,  quinine  may  be  obtained  as  an  amorphous 
powder  by  adding  ammonia. 

Quinine  crystallises  in  silky  needles,  melts  at  177°,  and  is 
only  very  sparingly  soluble  in  water ;  it  is  only  a  feeble  di- 
acid  base,  and  generally  forms  acid  salts,  such  as  the  sulphate, 
(C20H24]S"202)2,H2S04  +  8H20 ;  many  of  its  salts  are  soluble 
in  water,  and  much  used  in  medicine  as  tonics,  and  for  lower- 
ing the  temperature  in  cases  of  fever,  &c. 

Quinine  is  a  di-tertiary  base,  because  it  combines  with  methyl 
iodide  to  form  quinine  dimethiodide,  C20H24N202,(CH3I)2; 
it  is  a  derivative  of  quinoline,  because,  on  oxidation  with 
chromic  acid,  it  yields  qtiininic  acid  (methoxyquinoline-y-car- 

boxylic  acid). 

COOH 
/^\     ^ 

CH3 


Quinine  appears  to  be  methoxy-cinchonine,  and  that  it 
contains  one  methoxy-group,  has  been  demonstrated  by 
Zeisel's  method  (p.  486) ;  this  view  accords  with  the  fact 

*  Quinic  acid,  C6H7(OH)4-COOH,  crystallises  in  colourless  prisms,  and 
melts  at  162°.  It  is  a  derivative  of  benzoic  acid,  being,  in  fact,  hexahydro- 
tetrahydroxybenzoic  acid. 


ALKALOIDS.  493 

that,  whereas  cinchonine,  on  oxidation,  yields  quinoline-y- 
carboxylie  acid,  quinine  yields  the  rnethoxy-derivative  of 
this  acid :  in  spite,  however,  of  a  great  amount  of  laborious 
investigation,  the  constitution  of  quinine  is  still  an  unsolved 
problem. 

Tests  for  Quinine. — If  a  solution  of  a  salt  of  quinine  be 
mixed  with  chlorine-  or  bromine- water,  and  then  ammonia 
added,  a  highly  characteristic  emerald  green  colouration  is 
produced ;  quinine  is  also  characterised  by  the  fact  that 
dilute  solutions  of  its  salts  show  a  beautiful  light- blue 
fluorescence. 

Cinchonine,  C19H22N20,  accompanies  quinine  in  almost 
all  the  cinchona-barks,  and  is  present  in  some  kinds  (in 
the  bark,  China  Huanaco)  to  the  extent  of  2-5  per  cent. 

In  order  to  prepare  cinchonine,  the  mother-liquors  from  the 
crystals  of  quinine  sulphate  (see  above)  are  treated  with  soda,  and 
the  precipitate  dissolved  in  the  smallest  possible  quantity  of  boiling 
alcohol ;  the  crude  cinchonine,  which  separates  on  cooling,  is  further 
purified  by  converting  into  the  sulphate,  and  crystallising  this  salt 
from  water. 

Cinchonine  crystallises  in  colourless  prisms,  melts  at  250°, 
and  resembles  quinine  in  ordinary  properties;  its  salts,  for 
example,  are  antipyretics,  but  are  much  less  active  than 
those  of  quinine. 

Oxidising  agents,  such  as  nitric  acid  and  potassium  per- 
manganate, readily  attack  cinchonine,  converting  it  into  a 
variety  of  substances,  one  of  the  most  important  of  which  is 
cinchoninic  acid,  or  quinoline-y-carboxylic  acid, 

COOH 


The  formation  of  this  acid  not  only  proves  that  cinchonine 
is  a  quinoline-derivative,  but  also  shows  the  close  relationship 
existing  between  quinine  and  cinchouine  (see  above). 


494  ALKALOIDS. 

Strychnine,  C21H22N202,  and  brucine,  two  highly  poisonous 
alkaloids,  are  contained  in  the  seeds  of  Strychnos  nux  vomica 
and  of  Strychnos  Ignatii  (Ignatius'  beans),  but  they  are  usually 
extracted  from  the  former. 

Powdered  nux  vomica  is  boiled  with  dilute  alcohol,  the  filtered 
solution  evaporated  to  expel  the  alcohol,  and  treated  with  lead 
acetate  to  precipitate  tannin,  &c.  The  filtrate  is  then  treated  with 
hydrogen  sulphide  to  precipitate  the  lead,  and  the  filtered  solution 
mixed  with  magnesia  and  allowed  to  stand.  The  precipitated 
alkaloids  are  separated,  and  warmed  with  a  little  alcohol,  which 
dissolves  out  the  brucine  ;  the  residual  strychnine  is  further  purified 
by  recrystallisation  from  alcohol. 

The  alcoholic  solution  of  the  brucine— which  still  contains 
strychnine — is  evaporated,  and  the  residue  dissolved  in  dilute  acetic 
acid ;  this  solution  is  now  evaporated  to  dryriess  on  a  water-bath, 
during  which  process  the  strychnine  acetate  decomposes,  with  loss 
of  acetic  acid  and  separation  of  the  free  base.  The  stable  brucine 
acetate  is  dissolved  again  by  adding  water,  the  filtered  solution 
treated  with  soda,  and  the  precipitated  brucine  purified  by  re- 
crystallisation  from  dilute  alcohol. 

Strychnine  crystallises  in  beautiful  rhombic  prisms,  and 
melts  at  284° ;  although  it  is  very  sparingly  soluble  in  water 
(1  part  in  4000  at  15°),  its  solution  possesses  an  intensely 
bitter  taste,  and  is  very  poisonous.  Strychnine  is,  in  fact, 
one  of  the  most  poisonous  alkaloids,  half  a  grain  of  the 
sulphate  having  caused  death  in  twenty  minutes. 

Although  strychnine  contains  two  atoms  of  nitrogen,  it  is, 
like  brucine,  only  a  mon-acid  base,  forming  salts,  such  as  the 
hydrochloride,  C21H22N202,HC1,  with  one  equivalent  of  an 
acid  ;  many  of  the  salts  are  soluble  in  water.  It  is,  further- 
more, a  tertiary  base,  because  it  combines  with  methyl  iodide 
to  form  strychnine  methiodide,  C21H22N202,CH3I. 

When  distilled  with  potash,  strychnine  yields,  among  other 
products,  quinoline ;  probably,  therefore,  it  is  a  derivative  of 
this  base. 

Test  for  Strychnine. — Strychnine  is  very  readily  detected, 
as  it  shows  many  characteristic  reactions,  of  which  the  follow- 
ing is  the  most  important :  When  a  small  quantity  of  powdered 


ALKALOIDS.  495 

strychnine  is  placed  in  a  large  porcelain  basin,  a  little  con- 
centrated sulphuric  acid  added,  and  then  a  little  powdered 
potassium  bichromate  dusted  over  the  liquid,  an  intense  violet 
solution,  which  gradually  becomes  bright-red,  and  then  yellow, 
is  produced. 

Brucine,  C93H96N204,  crystallises  in  colourless  prisms,  with 
4  mols..  H20,  and  melts  at  178°.  It  is  more  readily  soluble 
in  water  and  in  alcohol  than  strychnine,  and,  although  very 
poisonous,  it  is  not  nearly  so  deadly  as  the  latter  (its  physio- 
logical effect  being  only  about  ^jth  of  that  of  strychnine). 
Although  it  contains  two  atoms  of  nitrogen,  brucine,  like 
strychnine,  is  a  mon-acid  base.  The  hydrochloride,  for  ex- 
ample, has  the  composition  C23H26N204,  HClj  it  is  also  a 
tertiary  base,  because  it  combines  with  methyl  iodide,  to  form 
brucine  metliiodide,  C23H26N204,  CH3I. 

Test  for  Brucine. — When  a  solution  of  a  brucine  salt  is 
treated  with  nitric  acid,  a  deep  brownish-red  colouration  is 
obtained,  and,  on  warming,  the  solution  becomes  yellow;  if 
now  stannous  chloride  be  added,  an  intense  violet  colouration 
is  produced. 

This  colour  reaction  serves  as  a  delicate  test,  both  for 
brucine  and  for  nitric  acid,  as  it  may  be  carried  out  with  very 
small  quantities. 

Alkaloids  contained  in  Opium. 

The  juice  of  certain  kinds  of  poppy-heads  (Papaver  somni- 
ferum)  contains  a  great  variety  of  alkaloids,  of  which  morphine 
is  the  most  important,  but  codeine,  narcotine,  theba'ine,  and 
papaverine  may  also  be  mentioned.  All  these  compounds  are 
present  in  the  juice  in  combination  with  meconic  acid*  and 
partly  also  with  sulphuric  acid.  When  incisions  are  made  in 

*  Meconic  acid,  C5HO2(OH)(COOH)2,  is  a  hydroxydiearboxylic  acid  be- 
longing to  the  fatty  series.  It  crystallises  with  three  molecules  of  water, 
and  gives,  with  ferric  chloride,  an  intense  dark-red  colouration.  In  cases 
of  suspected  opium-poisoning  this  acid  is  always  tested  for,  owing  to  the 
ease  with  which  it  can  be  detected  by  this  colour  reaction. 


496  ALKALOIDS. 

the  poppy-heads,  and  the  juice  which  exudes  is  collected 
and  left  to  dry,  it  assumes  a  pasty  consistency,  and  is  called 
opium.  An  alcoholic  tincture  of  opium,  containing  about  1 
grain  of  opium  in  15  minims,  is  known  as  laudanum. 

Preparation  of  Morphine. — Opium  is  extracted  with  hot  water, 
the  extract  boiled  with  milk  of  lime,  and  filtered  from  the  precipi- 
tate, which  contains  the  meconic  acid,  and  all  the  alkaloids,  except 
morphine.  The  filtrate  is  then  concentrated,  digested  with 
ammonium  chloride  until  ammonia  ceases  to  be  evolved  (to  convert 
any  lime  present  into  soluble  calcium  chloride),  and  allowed  to  stand 
for  some  days ;  the  morphine,  which  separates,  is  collected  and 
purified  by  recrystallisation  from  fusel  oil  (part  i.  p.  99). 

Morphine,  C17H19N03,  crystallises  in  colourless  prisms,  with 
1  mol.  H20,  and  is  only  slightly  soluble  in  water  and  cold 
alcohol,  but  dissolves  readily  in  potash  and  soda,  from  which 
it  is  reprecipitated  on  the  addition  of  acids ;  it  has,  in  fact, 
the  properties  of  a  phenol.  At  the  same  time,  it  is  a  mon- 
acid  base,  and  forms  well-characterised  salts  with  acids.  The 
hydrochloride,  Cl7H19N03,HCl  +  3H20,  crystallises  from 
water  in  colourless  needles,  and  is  the  salt  most  commonly 
employed  in  medicine.  Morphine  has  a  bitter  taste,  and  is 
excessively  poisonous,  one  grain  of  the  hydrochloride  having 
been  found  sufficient  to  cause  death  ;  on  the  other  hand,  the 
system  may  become  so  accustomed  to  the  habitual  use  of 
opium  that,  after  a  time,  very  large  quantities  may  be  taken 
daily  without  fatal  effects. 

Morphine  hydrochloride  is  extensively  used  in  medicine  as 
a  soporific,  especially  in  cases  of  intense  pain,  which  it  relieves 
in  a  remarkable  manner. 

Tests  for  Morphine. — Morphine  has  the  property  of  liberat- 
ing iodine  from  a  solution  of  iodic  acid.  If  a  little  iodic 
acid  be  dissolved  in  water,  and  a  few  drops  of  a  solution 
of  morphine  hydrochloride  added,  a  brownish  colouration  is  at 
once  produced,  owing  to  the  liberation  of  iodine,  and,  on 
adding  some  of  the  solution  to  starch-paste,  the  well-known 
deep-blue  colouration  is  obtained. 

A  solution  of  morphine,  or  of  a  morphine  salt,  gives  a  deep- 


ALKALOIDS.  497 

blue  colouration  with  ferric  chloride,  but,  perhaps,  the  most 
delicate  test  for  the  alkaloid  is  the  following  :  If  a  trace  of 
morphine  be  dissolved  in  concentrated  sulphuric  acid,  the 
solution  kept  for  15  hours,  and  then  treated  with  nitric  acid, 
it  gives  a  bluish-violet  colour,  which  changes  to  blood-red. 
This  reaction  is  very  delicate,  and  is  well  shown  by  0-01 
milligramme  of  morphine. 

The  constitution  of  morphine  is  still  undetermined,  but  that 
it  is  a  tertiary  base  is  proved  by  the  fact  that,  when  treated 
with  methyl  iodide,  it  yields  morphine  methiodide, 

C1YH19N03,CH3I. 

Morphine  contains  two  hydroxyl-groups,  one  of  which  is  phenolic, 
the  other  alcoholic.  The  third  atom  of  oxygen  present  in  the  mole- 
cule is  not  ketonic  (that  is,  present  as  >>CO) ;  it  must,  therefore, 
be  combined  with  two  carbon  atoms  -C— O— C-  (as  in  ordinary 
ether).  It  is  to  the  presence  of  the  phenolic  hydroxyl-group  that 
morphine  owes  its  property  of  dissolving  in  alkalies,  and  giving  a 
blue  colour  with  ferric  chloride. 

If  the  base  be  heated  with  potash  and  methyl  iodide,  methyl- 
morphine,  C17H17NO(OCH3)-OH,  is  produced,  a  substance  which  is 
identical  with  codeine,  an  alkaloid  which  accompanies  morphine 
in  opium.  Codeine  is  insoluble  in  alkalies,  and  is,  therefore,  not  a 
phenol  ;  it  behaves,  however,  like  an  alcohol,  and  gives,  with  acetic 
anhydride,  acetylcodeine,  C17H17NO(OCH3)-C2H302. 

It  is  very  remarkable  that  morphine  is  a  derivative  of  phenan- 
threne,  as  derivatives  of  this  hydrocarbon  are  very  seldom  met  with 
in  nature.  If  morphine  be  distilled  with  zinc-dust,  a  considerable 
quantity  of  this  hydrocarbon  is  obtained,  together  with  pyridine, 
quinoline,  and  other  substances. 

Alkaloids  related  to  Uric  Acid. 

Caffeine,  theine,  or  methyltheobromine,  C8H10N402,  occurs 
in  coffee-beans  (-|  per  cent.),  in  tea  (2  to  4  per  cent.),  in 
kola-nuts  (2-5  per  cent.),  and  in  other  vegetable  products. 

Tea  (1  part)  is  macerated  with  hot  water  (4  parts),  milk  of  lime 
(1  part)  added,  and  the  whole  evaporated  to  dry  ness  on  a  water- 
bath  ;  the  caffeine  is  then  extracted  from  the  residue  by  means  of 
chloroform,  the  extract  evaporated,  and  the  crude  base  purified  by 
recrystallisation  from  water. 


498  ALKALOIDS. 

Caffeine  crystallises  in  long,  colourless  needles,  with  1  moL 
H20,  melts  at  225°,  and  at  higher  temperatures  sublimes  un- 
decomposed ;  it  has  a  bitter  taste,  and  is  sparingly  soluble 
in  cold  water  and  alcohol.  Caffeine  is  a  feeble  base,  and 
forms  salts  only  with  strong  acids;  the  hydrochloride, 
C8H10N"402,HC1,  is  at  once  decomposed  on  treatment  with 
water,  with  separation  of  the  base. 

The  constitution  of  caffeine  has  been  determined  by  E. 
Fischer,  who  has  shown  that  this  substance  and  uric  acid  are 
very  closely  allied ;  caffeine  is,  therefore,  an  example  of  an 
alkaloid  which  is  not  a  derivative  of  pyridine  or  quinoline. 

Tests  for  Caffeine. — If  a  trace  of  caffeine  be  evaporated  with 
concentrated  nitric  acid,  it  gives  a  yellow  residue  (amalinic 
acid),  which,  on  the  addition  of  ammonia,  becomes  intensely 
violet  (murexide  reaction) ;  this  reaction  is  also  shown  by  uric 
acid  (part  i.  p.  292).  A  solution  of  caffeine  in  chlorine  water 
yields,  on  evaporation,  a  yellowish-brown  residue,  which  dis- 
solves in  dilute  ammonia,  with  a  beautiful  violet-red  colouration. 

Theobromine,  C7H8N402,  occurs  in  cocoa-beans,  from  which  it  may 
be  obtained  by  treatment  with  lime,  and  extraction  with  alcohol.  It 
crystallises  from  water,  and  shows  the  greatest  resemblance  to 
caffeine  in  properties ;  the  latter  is,  in  fact,  methyltheobromine,  and 
may  be  obtained  directly  from  theobromine  in  the  following  way  : 

Theobromine  contains  an  >NH  group,  the  hydrogen  of  which  is 
readily  displaced  by  metals  (as  in  succinimide,  part  i.  p.  238),  and 
when  treated  with  an  ammoniacal  silver  nitrate  solution,  it  yields 
silver  theobromine.  This  substance  interacts  readily  with  methyl 
iodide  with  formation  of  caffeine, 

C7H7N402Ag  +  CHSI  =  C7H7N402-CH3  +  Agl. 

Silver  Theobromine.  Caffeine. 

The  relationship  between  uric  acid,  theobromine,  and  caffeine  is 
expressed  by  the  following  graphic  formulae  : 

/NH.CO-C.NHv  /NH.CO.C.N(CH3k 

C0<  ||          >CO  C0<  >CH 

NNH C-NH/  XN(CH3)-C W 

Theobromine. 

xN(CH3)-CO.C.N(CH3)x 
C0<  !!.  V/CH 

^"~ " i\ 


N 


N(CH3) 


Caffeine. 


ALKALOIDS.  499 

Antipyrine,  Katrine,  and  Tkalline. 

These  three  nitrogenous  compounds,  which  do  not  occur  in 
nature,  may  be  briefly  described  here  as  examples  of  what 
may  be  termed  '  artificial  alkaloids ; '  they  are  employed  in 
medicine,  as  substitutes  for  quinine,  for  lowering  the  body 
temperature  in  cases  of  fever. 

Antipyrine,  C11H12N20,   was   first  obtained  by  Knorr  by 
treating    ethyl   acetoacetate   (part   i.    p.    189)   with    phenyl- 
hydrazine  (p.    376),   and  then  heating  the  product  (phenyl- 
methylpyrazolone)  with  methyl  iodide, 
CH8.CO.CH9.COOC2H5  +  C6H5-NH.NH9  = 

C10H10N20  +  C2H5-OH  +  H20 
C10H10N20  +  CH3I  =  CnH12N20,HI. 

It  is  a  colourless,  crystalline  compound,  melts  at  113°,  and  is 
readily  soluble  in  water  and  alcohol ;  it  is  a  strong  mon-acid 
base,  and  its  salts  dissolve  freely  in  water.  Its  aqueous  solu- 
tion gives  a  deep-red  colouration  with  ferric  chloride,  and  a 
bluish-green  colouration  with  nitrous  acid. 

Kairine,  or  hydroxymethyltetrahydroquinoline, 


may  be  obtained  indirectly  from  o-amidophenol,  which  is  first 
converted  into  hydroxyquinoline  by  Skraup's  reaction  (p.  482); 
this  product  is  then  reduced  with  tin  and  hydrochloric  acid, 
and  the  tetrahydrohydroxyquinoline  thus  obtained  is  converted 
into  its  methyl-derivative  by  treating  it  with  methyl  iodide. 

Kairine  is  a  crystalline  compound,  melting  at  114°.  It  is  a 
strong  base,  and  forms  crystalline  salts,  of  which  the  hydro- 
chloride,  C10H13IST0,HC1  +  H20,  is  used  in  medicine. 

Thalline,  or  methoxytetrahydroquinoline, 

;H0.CHfl 


is  isomeric  with  kairine,  and  is  obtained  by  reducing  the 
methoxyquinoline  which  is  prepared  from  p-methoxy aniline, 
C6H4(OCH3)-NH2,  by  Skraup's  reaction  ;  it  is  a  crystalline 


500  ALKALOIDS. 

compound,  melting  at  42°,  and  is  used  in  the  form  of  its 
sulphate  or  tartrate.  With  ferric  chloride  and  other  oxidising 
agents  it  gives  a  green  precipitate. 

Antifebrin,  or  acetanilide,  another  important  febrifuge,  has 
already  been  described  (p.  362). 

Choline,  Beta'ine,  Neurine,  and  Taurine. 

Certain  nitrogenous  substances  which  occur  in  the  animal 

kingdom  may  also  be  referred  to  in  this  chapter,  because  they 

are  basic  compounds  of  great  physiological  importance  ;  they 

really  belong,  however,  to  different  classes  of  the  fatty  series. 

Choline,  or  hydroxyethyltrimethylammonium  hydroxide, 
OH.CH2.CH2.N(CH3)3.OH,  occurs  in  the  blood,  bile,  brain- 
substance,  yolk  of  egg,  and  in  other  parts  of  animal  organisms, 
usually  -in  the  form  of  lecithin  (a  compound  of  choline, 
glycerol,  phosphoric  acid,  and  various  fatty  acids)  ;  it  also 
occurs  in  mustard  and  in  hops.  It  may  be  prepared  syntheti- 
cally by  warming  trimethylamine  with  ethylene  oxide  (part  i. 
p.  223)  in  aqueous  solution, 

N(CH3)3  +  C2H40  +  H20  =  C5H15N02, 
It  is  a  crystalline,  very  hygroscopic,  strongly  basic  substance, 
its  aqueous  solution  having  an  alkaline  reaction,  and  absorb- 
ing carbon  dioxide  from  the  air  ;   when  treated  with  hydro- 
chloric acid  it  yields  the  corresponding  chloride, 
OH.CH9.CH2-N(CH3)3.OH  +  HC1  = 

OH.CH2.CH2.N(CH3)3C1  +  H2O, 

but  when  boiled  with   water  the  base  is  decomposed  into 
glycol  and  trimethylamine. 

Betaine,  C5HnN02,  is  formed  when  choline  undergoes  mild 
oxidation  ;  the  acid,  which  is  first  produced  by  the  conversion 
of  the  -CH2'OH  group  into  carboxyl, 


33  +  20  =  COOH.CH2.N<s3  +  H20, 

CH2  —  COv 
loses  one  molecule  of  water,  forming  betaine,      " 


ALKALOIDS.  501 

salt-like  compound,  which  has  a  neutral  reaction,  a  somewhat 
sweet  taste,  and  crystallises  from  dilute  alcohol  with  1  mol. 
H20. 

When  treated  with  hydrochloric  acid,  betaine  is  converted 
into  the  chloride,  COOH-CH2.N(CH3)3C1,  and  this  compound 
may  also  be  obtained  synthetically  by  heating  trimethylamine 
with  chloracetic  acid.  Betaine  occurs  in  beet-juice,  and  is 
present  in  large  quantities  in  the  mother-liquors  obtained  in 
the  preparation  of  beet-sugar. 

Neurine,  or  vinyltrimethylammonium  hydroxide, 

CH2:CH.N(CH3)3-OH, 

can  be  obtained  by  heating  choline  with  hydriodic  acid,  and 
then  treating  the  product  with  silver  hydroxide, 

OH.CH2.CH2.N(CH3)3-OH  +  2HI  = 

CH2LCH2.N(CH3)3I  +  2H20 

CH2LCH2.N(CH3)3I  +  2AgOH  = 

CH2:CH-N(CH3)3.OH  +  2AgI  +  H,0  ; 

it  is  formed,  together  with  choline  and  numerous  other  bases, 
during  the  putrefaction  of  animal  albuminoid  matter.* 

Neurine  is  only  known  in  solution  as  a  strongly  basic,  very 
soluble,  and  exceedingly  poisonous  substance,  but  some  of  its 
salts,  as,  for  example,  the  chloride,  CH2:CH-]NT(CH3)3C1,  are 
crystalline. 

Taurine,  or  amidoethylsul phonic  acid,  NH2-CH2-CH2-S03H, 
occurs  in  the  combined  state  in  ox-gall  and  in  many  other 
animal  secretions.  It  crystallises  in  colourless  prisms,  melts 
and  decomposes  at  about  240°,  and  is  readily  soluble  in  water, 
but  insoluble  in  alcohol ;  it  has  a  neutral  reaction,  and  is 
only  a  feeble  acid,  because  the  presence  of  the  amido-group 
neutralises  the  effect  of  the  sulphonic  group  to  such  an  extent 
that  it  forms  salts  only  with  strong  bases.  When  treated 
with  nitrous  acid,  the  amido-group  is  displaced  by  hydroxyl, 

*  The  bases  produced  during  the  putrefaction  of  animal  albuminoid 
matter  are  known  collectively  as  ptomaines,  and  many  of  them  are  highly 
poisonous. 


502  ALKALOIDS. 

just  as  in  the   case  of  primary  amines,   and   hydroxyethyl- 
sulphonic  acid  (isethionic  acid)  is  formed, 


jm2.CH2.CH2.S03H  +  HO-NO  = 


OH.CH2-CH2.S03H  +  N2  +  H20  ; 

the  last-named  compound  is  one  of  the  few  examples  of  fatty 
sulphonic  acids. 


CHAPTER    XXXIV. 

DYES    AND    THEIR   APPLICATION. 

Although  nearly  all  fatty  compounds,  and  the  majority  of 
those  belonging  to  the  aromatic  series,  are  colourless,  most  of 
the  principal  dyes  used  at  the  present  day  are  aromatic  com- 
pounds, the  primary  source  of  which  is  coal-tar. 

That  a  dye  must  be  a  coloured  substance  is,  of  course, 
obvious,  but  a  coloured  substance  is  not  necessarily  a  dye,  in 
the  ordinary  sense  of  the  word,  unless  it  is  also  capable  of 
fixing  itself,  or  of  being  fixed,  in  the  fabric  to  be  dyed,  in 
such  a  way  that  the  colour  is  not  removed  by  rubbing  or  by 
washing  with  water;  azobenzene,  for  example,  is  intensely 
coloured,  but  it  would  not  be  spoken  of  as  a  dye,  because  it 
does  not  fulfil  the  second  condition. 

True  dyes,  in  the  sense  just  defined,  may  be  roughly  divided 
into  two  classes  with  respect  to  their  behaviour  with  a  given 
fabric :  (a)  Those  which  fix  themselves  on  the  fabric,  and 
(b)  those  which  do  so  only  with  the  aid  of  a  mordant,, 

If  a  piece  of  silk  or  wool  be  dipped  into  a  solution  of  picric 
acid,  it  is  dyed  yellow,  and  the  colour  is  not  removed  on 
subsequently  washing  with  water,  but  is  fixed  in  the  fibre. 
If,  however,  a  piece  of  calico  or  other  cotton  material  be 
treated  in  the  same  way,  the  picric  acid  does  not  fix  itself, 
and  is  completely  removed  on  washing  with  water.  A  given 
substance  may,  therefore,  be  a  dye  for  certain  materials,  but 
not  for  others;  the  animal  fabrics,  silk  and  wool,  fix  picric 


DYES    AND    THEIR   APPLICATION.  503 

acid,  and  are  dyed  by  it,  but  the  vegetable  fabric,  cotton, 
does  not — a  behaviour  which  is  repeatedly  met  with  in  the 
case  of  other  colouring  matters  (see  below). 

Now,  since  picric  acid  is  soluble  in  water,  it  is  evident  that 
it  must  have  undergone  some  change  when  brought  into 
contact  with  the  silk  or  wool,  otherwise  it  would  be  dissolved 
out  of  the  fabric  on  washing  with  water.  Materials  such  as 
wool,  cotton,  silk,  &c.,  consist  of  minute  fibres,  which  may 
be  very  roughly  described  as  long,  cylindrical,  or  flattened 
tubes  (except  in  the  case  of  silk,  the  fibres  of  which  are  solid), 
the  walls  of  which,  like  parchment  paper  and  animal  mem- 
brane, allow  of  the  passage  of  water  and  of  dissolved  crystalloids 
by  diffusion,  but  not  of  colloid  substances,  or,  of  course,  of 
matter  in  suspension.  If,  therefore,  the  picric  acid  were 
present  in  the  fibre,  as  picric  acid,  it  would,  on  washing, 
rapidly  pass  into  the  water  by  diffusion ;  as  this  is  not  the 
case,  it  must  be  assumed  that  it  has  actually  combined  with 
some  substance  in  the  silk  or  wool,  and  has  been  converted 
into  a  yellow  compound,  which  is  either  insoluble  or  a  colloid. 

The  nature  of  the  insoluble  compound  formed  when  a  material 
is  dyed  in  this  way  is  not  known,  but  there  are  reasons  for  suppos- 
ing that  certain  constituents  of  the  fibre  unite  with  the  dye  to  form 
an  insoluble  salt.  This  seems  probable,  from  the  fact  that  nearly 
all  dyes  which  thus  fix  themselves  directly  on  the  fabric  are,  to 
some  extent,  either  basic  or  acid  in  character.  Azobenzene,  as 
already  mentioned,  is  not  a  dye,  probably,  because  it  is  a  neutral 
substance ;  if,  however,  some  group,  such  as  an  amido-,  hydroxyl-, 
or  sulphonic-group,  which  confers  basic  or  acid  properties,  be  intro- 
duced into  the  molecule  of  azobenzene,  then  the  resulting  deriva- 
tive is  a  dye,  because  it  has  the  property  of  combining  directly  with 
the  fibres  of  certain  materials  (compare  p.  522). 

Another  fact  which  leads  to  the  same  conclusion  may  be  quoted. 
Certain  dyes— as,  for  example,  rosaniline— are  salts  of  bases  which 
are  themselves  colourless,  and  yet  some  materials  may  be  dyed 
simply  by  immersion  in  colourless  solutions  of  these  bases,  the 
same  colour  being  obtained  as  with  the  coloured  salt  (that  is,  the  dye 
itself) ;  this  can  only  be  explained  by  assuming  that  some  con- 
stituent of  the  fibre  combines  with  the  colourless  base,  forming 
with  it  a  salt  of  the  same  colour  as  the  dye. 


504  DYES   AND   THEIR  APPLICATION. 

Some  fibres,  especially  silk  and  wool,  seem  to  contain  both  acid 
and  basic  constituents,  as  they  are  often  dyed  directly  both  by 
basic  and  by  acid  dyes ;  cotton,  on  the  other  hand,  seems  to  be 
almost  free  from  both,  as,  except  in  rare  cases,  it  does  not  combine 
with  colouring  matters. 

Granting,  then,  that  the  fixing  of  a  dye  within  the  fibre  is 
the  result  of  its  conversion  into  some  insoluble  compound, 
it  seems  reasonable  to  suppose  that,  even  if  a  colour- 
ing matter  be  incapable  of  fixing  itself  in  the  fibre  of  the 
material,  it  might  still  be  employed  as  a  dye,  provided  that, 
after  it  had  once  passed  through  the  walls  of  the  fibre,  it 
could  be  there  converted  into  some  insoluble  compound  by 
other  means;  this  principle  is  applied  in  the  case  of  dyes 
of  the  second  class,  which  are  fixed  in  the  material  with  the 
aid  of  mordants. 

Mordants  are  substances  which  (usually  after  first  under- 
going some  preliminary  change)  combine  with  dyes,  forming 
insoluble  coloured  compounds ;  the  colour  of  the  dyed  fabric 
in  such  cases  depends,  of  course,  on  that  of  the  compound 
thus  produced,  and  not  on  that  of  the  dye  itself,  so  that  by 
using  different  mordants,  different  shades  or  colours  are 
obtained. 

As  an  example  of  dyes  of  the  second  class,  alizarin 
may  be  taken,  as  it  illustrates  very  clearly  the  use  of 
mordants. 

If  a  piece  of  calico  be  dipped  into  a  solution  of  alizarin, 
it  is  coloured  yellow,  but  the  colour  is  not  fixed,  and  is  easily 
got  rid  of  again  on  washing  with  soap  and "  water ;  if,  how- 
ever, a  piece  of  calico,  which  has  been  previously  mordanted 
with  a  suitable  aluminium  salt  (in  the  manner  described 
below),  be  treated  in  the  same  way,  it  is  dyed  a  fast  red,  the 
alizarin  having  combined  with  the  aluminium  salt  in  the  fibre 
to  form  a  red  insoluble  compound ;  if,  again,  the  calico  had 
been  mordanted  with  a  ferric  salt  instead,  it  would  have  been 
dyed  a  fast  dark  purple. 

Substances    very   frequently    employed    as    mordants    are 


DYES    AND    THEIR    APPLICATION.  505 

certain  salts  of  iron,  aluminium,  chromium,  and  tin,  more 
especially  those,  such  as  the  acetates,  sulphocyanides,  and 
alums,  which  undergo  decomposition  on  treatment  with  water 
or  with  steam,  yielding  either  an  insoluble  basic  salt  or  an 
insoluble  metallic  hydroxide. 

The  process  of  mordanting  usually  involves  two  operations : 
firstly,  the  fabric  is  passed  through,  or  soaked  in,  a  solution 
of  the  mordant,  in  order  that  its  fibres  may  become  impreg- 
nated with  the  metallic  salt ;  secondly,  the  fabric  is  treated 
in  such  a  way  that  the  salt  is  decomposed  within  the  fibres, 
and  there  converted  into  some  insoluble  compound. 

This  second  operation,  the  fixing  of  the  mordant,  so  that  it 
will  not  be  washed  out  when  the  fabric  is  brought  into  the 
dye-bath,  is  accomplished  in  many  ways.  One  of  the 
simplest  is  to  pass  the  mordanted  material  through  a  solution 
of  some  weak  alkali  (ammonia,  sodium  carbonate,  lime)  or  of 
some  salt,  such  as  sodium  phosphate  or  arsenate,  which  inter- 
acts with  the  metallic  salt  in  the  fibre,  forming  an  insoluble 
metallic  hydroxide,  phosphate,  arsenate,  &c.  Another  method, 
applicable  more  especially  in  the  case  of  mordants  which  are 
salts  of  volatile  acids,  consists  in  exposing  the  fabric  to  the 
action  of  steam,  at  a  suitable  temperature ;  under  these  con- 
ditions the  metallic  salt  dissociates,  the  acid  volatilises  with 
the  steam,  and  an  insoluble  hydroxide  or  basic  salt  remains  in 
the  fibre. 

In  the  case  of  silk  and  woollen  fabrics,  the  operations  of 
mordanting  and  fixing  the  mordant  may  often  be  carried  out 
simultaneously,  by  soaking  the  materials  in  a  boiling  dilute 
solution  of  the  mordant ;  under  these  conditions,  the  metallic 
salt  is  partially  dissociated,  and  deposited  in  the  fibre  in  an 
insoluble  form ;  silk  may  sometimes  be  simply  soaked  in  a 
cold,  concentrated  solution  of  the  mordant,  and  then  washed 
with  water  to  cause  the  dissociation  of  the  metallic  salt. 

In  cases  where  only  parts  of  the  fabric  are  to  be  dyed,  as, 
for  example,  in  calico-printing,  the  solution  of  the  mordant  is 
mixed  with  the  dye,  and  with  some  thickening  substance, 


506  DYES  AND  THEIH  APPLICATION. 

such  as  starch,  dextrin,  gum,  &c.,  and  printed  on  the  fabric 
in  the  required  manner,  the  thickening  being  used  to  prevent 
the  mordant  spreading  to  other  parts  ;  during  the  subsequent 
steaming  process,  the  metallic  hydroxide  which  is  produced 
combines  with  and  fixes  the  dye. 

All  these  processes  are  identical  in  principle,  the  object 
being  to  deposit  some  insoluble  metallic  compound  within  the 
fibre  ;  when,  now,  the  mordanted  material  is  treated  with  a 
solution  of  a  suitable  dye,  the  latter  unites  with  the  metallic 
hydroxide,  forming  a  coloured  compound  which  is  fixed  in 
the  fibre.  The  coloured  substances  produced  by  the  combina- 
tion of  a  dye  with  a  metallic  hydroxide  are  termed  lakes,  and 
those  dyes  which  form  lakes  are  called  acid  dyes. 

Tannin  (p.  440)  is  an  example  of  a  different  class  of 
mordants  —  namely,  of  those  which  are  employed  with  basic 
dyes,  such  as  malachite  green  (p.  509)  and  rosaniline  (p.  513)  : 
its  use  depends  on  the  fact  that,  being  an  acid,  it  combines 
with  dyes  of  a  basic  character,  forming  with  them  insoluble 
coloured  salts  (tannates),  which  are  thus  fixed  in  the  fibre. 
The  fabric  is  mordanted  by  first  passing  it  through  a  solution 
of  tannin,  and  then  through  a  weak  solution  of  tartar  emetic, 
or  stannic  chloride,  which  converts  the  tannin  into  an  insoluble 
antimony,  or  tin  tannate,  and  thus  fixes  it  in  the  fibre. 

All  colouring  matters  are  converted  into  colourless  compounds 
on  reduction,  and  in  many  cases  such  a  radical  change  in 
composition  takes  place,  that  the  reduction  product  cannot  be 
directly  reconverted  into  the  dye  by  oxidation  ;  a  nitro-group, 
for  example,  may  be  reduced  to  an  amido-group,  or 
a  hydroxyl-group  may  be  displaced  by  hydrogen,  or  the 
molecule  may  be  resolved  into  two  simpler  molecules,  as  in 
the  case  of  amidoazobenzene,  which,  when  treated  with 
powerful  reducing  agents,  yields  aniline  and  £>-phenylene- 
diamine, 


C6H5.N:N-C6H4.NH2  +  4H  =  C6 

In  very    many    cases,    however,    the   colourless   reduction 


DYES   AND    THEIR   APPLICATION.  507 

product  differs  from  the  dye  in  composition,  simply  in 
containing  two  or  more  additional  atoms  of  hydrogen,  and 
may  be  readily  reconverted  into  the  dye  by  oxidising  agents ; 
such  reduction  products  are  called  leucocompounds. 

Amidoazobenzene,  for  example,  the  hydrochloride  or 
oxalate  of  which  is  the  dye  aniline  yellow  (p.  524),  on  treat- 
ment with  mild  reducing  agents,  such  as  zinc-dust  and 
acetic  acid,  yields  amidohydrazobenzene,  which  is  only 
slightly  coloured, 

C6H5.N:tf.C6H4.NH2  +  2H  =  C6H5-NH.NH.C6H4.NH2. 

The  last-named  substance  is  readily  oxidised  on  shaking  its 
alcoholic  solution  with  precipitated  (yellow)  mercuric  oxide, 
with  regeneration  of  amidoazobenzene,  and  is,  therefore,  leuco- 
amidoazobenzene  ;  many  examples  of  leuco-compounds  will  be 
met  with  in  the  following  pages. 

When  an  insoluble  dye  yields  a  soluble  leuco-compound, 
which  is  very  readily  reconverted  into  the  dye  on  oxidation, 
it  may  be  applied  to  fabrics  in  a  special  manner,  as,  for 
example,  in  the  case  of  dyeing  with  indigo  blue.  Indigo 
blue,  C16H10N202  (p.  527),  is  insoluble  in  water,  but  on 
reduction  it  is  converted  into  a  readily  soluble  leuco-base, 
C16H12N"202,  known  as  indigo  ivliite :  in  dyeing  with  indigo, 
a  solution  of  indigo  white  is  prepared  by  reducing  indigo, 
suspended  in  water,  with  grape-sugar  and  soda,  or  ferrous 
sulphate  and  soda,  and  the  fabric  is  then  passed  through 
this  solution,  whereupon  the  indigo  white  diffuses  through 
the  walls  into  the  fibres ;  on  subsequent  exposure  to  the  air 
the  indigo  white  is  reconverted  into  indigo  blue  by  oxidation, 
and  the  insoluble  dye  is  thus  fixed  in  the  fabric. 

Some  of  the  more  important  dyes  will  now  be  described  : 
as,  however,  it  would  be  impossible  to  discuss  fully  the 
constitutions  of  these  compounds,  it  must  be  understood  that 
the  formulae  employed  in  the  following  pages  are  those  com- 
monly accepted,  and  that  most  of  them  have  been  satis- 
factorily established. 


508  DYES   AND   THEIR   APPLICATION. 

Derivatives  of  Triphenylmethane. 

Triphenylmethane,  C6H5-CH(C6H5)2  (p.  340),  or,  more 
strictly  speaking,  triphenyl  carbinol,  C6H5-C(C6H5)2-OH,  is 
the  parent  substance  of  a  number  of  dyes,  which  are  of  very 
great  technical  importance,  on  account  of  their  brilliancy : 
as  examples,  malachite  green,  pararosaniline,  and  rosaniline 
may  be  described. 

Three  distinct  classes  of  substances  are  constantly  met 
with  in  studying  the  tripheny  1m ethane  group  of  colouring 
matters — namely,  the  leuco-base,  the  colour-base,  and  the 
dye  itself. 

The  leuco-base  (p.  507)  is  an  amido-derivative  of  triphenyl- 
methane;  in  the  case  of  malachite  green,  for  example,  the 
leuco-base  is  tetramethyldiamidotriphenylmethane, 


The  colour-base  is  a  derivative  of  triphenyl  carbinol,  and  is 
produced  from  the  leuco-base  by  oxidation,  just  as  triphenyl 
carbinol  results  from  the  oxidation  of  triphenylmethane  (p. 
341)  ;  tetramethyldiainidotriphenyl  carbinol,  for  example,  is 
the  colour-base  of  malachite  green, 


Both  the  leuco-base  and  the  colour-base  are  usually  colour- 
less, and  the  latter  also  yields  colourless,  or  only  slightly 
coloured,  salts  on  treatment  with  cold  acids;  when  warmed 
with  acids,  however,  the  colour-base  is  at  once  converted 
into  highly  coloured  salts,  which  constitute  the  dye,  water 
being  eliminated, 

C23H26N20  +  HC1  =  C23H25N2C1  +  H20. 

Malachite  Green  Base.          Chloride  of  Malachite  Green. 

This  loss  of  water  must  be  assumed  to  be  due  to  com- 
bination taking  place  between  the  hydroxyl-group  and  the 
hydrogen  atom  of  the  acid  employed,  and  the  conversion  of 


DYES    AND    THEIR    APPLICATION.  509 

the  colourless,  into  the  coloured,  salt  may  be  expressed  in  the 
following  way  : 


TT     p/nTT^^6la4*^^/ra3/2  —  P  W     rV^e^-M^J-lg^  i_  IT  O 

tlg-L-^UJlJ^^  TT  .ivr/piT  »    trpi  —  U6H5-l^n  TT   _xr/pir  vni  +  ±l2U. 


This  change  resembles  the  conversion  of  colourless  hydro- 
quinone  into  highly  coloured  quinone  (and  also  that  of  p- 
amidophenol  into  quinone-chlorimide,  p.  416),  as  will  be  more 
readily  understood  if  it  be  represented  thus : 

C6H5.C(OH).C6H4.X(CH3)2        C6H5.C.C6H4.N(CH3)2 


(CH3)2N,  HC1 

Hydrochloride  of  Colour-base.  Chloride  of  Malachite  Green. 

Exactly  similar  changes  may  be  assumed  to  take  place  in  the 
formation  of  the  pararosaniline  and  rosaniline  dyes,  and,  in 
fact,  in  the  case  of  many  other  colouring  matters,  some  of 
which  are  described  later. 

Malachite  green  (of  commerce)  is  a  double  salt,  formed  by  the 
combination  of  the  chloride  of  tetramethyldiamidotriphenyl 
carbinol  with  zinc  chloride,  and  the  first  step  in  its  manu- 
facture is  the  preparation  of  leuco-malachite  green  or 


Leuco-malachite  green  is  obtained  by  the  action  of 
dehydrating  agents,  generally  zinc  chloride,  on  a  mixture  of 
benzaldehyde  (1  mol.)  and  dimethylaniline  (2  mols.), 


C6H,CHO 


It  is  a  colourless,  crystalline   substance,  which,  when  treated 
with    oxidising    agents,     such    as    manganese'  dioxide    and 


510  DYES    AND  THEIR    APPLICATION. 

sulphuric  acid,  or  lead  dioxide  and  hydrochloric  acid, 
yields  tetrametliyldiamidotriplienyl  carbinol,  just  as  triphenyl- 
methane,  under  similar  circumstances,  yields  triphenyl  carbinol, 


This  oxidation  product  is  a  colourless  base,  and  dissolves 
in  cold  acids,  yielding  colourless  solutions  of  its  salts ; 
when,  however,  such  solutions  are  warmed,  the  colourless 
salts  decompose,  and  lose  one  molecule  of  water,  intensely 
green  solutions  of  the  dye  being  obtained ;  the  formation 
of  the  chloride,  for  example,  is  expressed  by  the  equation 

C23H26N20  +  HC1  =  C23H25N2C1  +  H20, 

and  its  double  salt,  with  zinc  chloride  (or  the  oxalate  of  the 
base),  constitutes  the  malachite  green  (Victoria  green,  benzal- 
dehyde  green)  of  commerce. 

Preparation  of  Malachite  Green. — Dimethylaniline(10  parts)  and 
benzaldehyde  (4  parts)  are  heated  with  zinc  chloride  (4  parts)  in  a 
porcelain  basin,  or  enamelled  iron  pot,  for  two  days  at  100°,  with 
constant  stirring  ;  the  product  is  then  submitted  to  distillation  in 
steam,  to  get  rid  of  the  unchanged  dimethylaniline,  and  allowed 
to  cool.  The  leuco-compound  is  now  separated  from  the  aqueous 
solution  of  zinc  chloride,  washed  with  water,  dissolved  in  as 
little  hydrochloric  acid  as  possible,  the  solution  diluted  consider- 
ably with  water,  and  the  calculated  quantity  of  freshly  pre- 
cipitated lead  peroxide,  (Pb02),  added.  The  filtered  dark-green 
solution  is  then  mixed  with  sodium  sulphate,  to  precipitate  any 
lead,  again  filtered,  and  the  colouring  matter  precipitated  in  the 
form  of  its  zinc  double  salt,  SC^HogNaCl^ZnCls  +  2H2O,  by 
the  addition  of  zinc  chloride  and  common  salt ;  this  salt  is  finally 
purified  by  recrystallisation. 

Malachite  green,  and  other  salts  of  the  base,  such  as  the 
oxalate,  2C23H24N2,3C2H204,  form  deep-green  crystals,  and 
are  readily  soluble  in  water ;  they  are  decomposed  by  alkalies, 
with  separation  of  the  colour-base,  tetramethyldiamidotri- 
phenyl  carbinol. 


DYES    AND    THEIR    APPLICATION.  511 

Malachite  green  dyes  silk  and  wool  directly  an  intense 
dark-bluish  green,  but  cotton  must  first  be  mordanted  with 
tannin  and  tartar  emetic  (p.  506),  and  then  dyed  in  a  bath 
gradually  raised  to  60°. 

Many  other  dyes,  closely  allied  to  malachite  green,  are  prepared 
by  condensing  benzaldehyde  with  tertiary  alkylanilines  (p.  366). 
Brilliant  green,  for  example,  is  finally  obtained  when  diethyl- 
aniline  is  employed  instead  of  dimethylaniline  in  the  above- 
described  process,  whereas  acid  green  is  obtained  from  benzal- 
dehyde and  ethylbenzylaniline,*  C6H5-N(C2H5)-C7H7,  in  a  similar 
manner.  The  salts  of  these  two  colouring  matters  are  very 
sparingly  soluble  in  water,  and,  therefore,  of  little  use  as  dyes  ; 
for  this  reason,  the  bases  are  treated  with  anhydrosulphuric  acid, 
and  thus  converted  into  a  mixture  of  readily  soluble  sulphonic 
acids,  the  sodium  salts  of  which  constitute  the  commercial  dyes. 
Silk  and  wool  are  dyed  in  a  bath  acidified  with  sulphuric  acid 
(hence  the  name  acid  green),  and  very  bright  greens  are  obtained, 
but  these  dyes  are  not  suitable  for  cotton. 

Pararosaniline  and  rosaniline  are  exceedingly  important 
dyes,  which,  like  malachite  green,  are  derived  from  triphenyl- 
methane.  Whereas,  however,  malachite  green  is  a  derivative 
of  cfo'a??izV?0-triphenylmethane,  the  rosanilines  are  all  triamido- 
triphenylmethane  derivatives,  as  will  be  seen  from  the  follow- 
ing table  : 


Triphenylmethane.  Tolyldiphenylmethane 

(MethyltriphenylmethaneX 


Leuco-pararosaniline  Leuco-rosaniline 

(Paraleucaniline).  (Leucaniline). 

Triamidotriphenylmethane.  Triamidotolyldiphenylmethane. 


4  -^"2 

Pararosaniline  Base.  Rosaniline  Base. 

Triainidotriphenyl  Carbinol.  Triamidotolyldiphenyl  Carbinol. 


L^"  XI   J.i 

Pararosaniline  Chloride.  Rosaniline  Chloride.  " 

Produced  by  treating  aniline  with  benzylchloride  and  ethyl  bromide 


512  DYES    AND    THEIR    APPLICATION. 

In  all  these  compounds,  the  amido-groups  have  been  proved 
to  be  in  the  jpara-position  to  the  methane  carbon  atom. 

Pararosaniline  (of  commerce)  is  the  chloride  of  triamido- 
triphenyl  carbinol,  a  base  which  is  most  conveniently  pre- 
pared by  oxidising  a  mixture  of  jp-toluidine  (1  mol.)  and 
aniline  (2  mols.)  with  arsenic  acid,  or  nitrobenzene  (compare 
rosaniline,  p.  513). 


NH2.C6H4.CH3  + 

2H0. 


Probably  the  j9-toluidine  is  first  oxidised  to  ^-amidobenzaldehyde, 
NH2-C6H4-CHO,  -which  then  condenses  with  the  aniline  (as  in  the 
case  of  the  formation  of  leuco-malachite  green),  to  form  leuco-para- 
rosaniline  ;  this  compound  is  then  converted  into  the  pararosaniline 
base  by  further  oxidation. 

The  salts  of  pararosaniline  have  a  deep  magenta  colour,  and 
are  soluble  in  warm  water;  they  dye  silk,  wool,  and  cotton, 
under  the  same  conditions  as  described  in  the  case  of 
malachite  green  ;  pararosaniline  is,  however,  not  so  largely 
used  as  rosaniline. 

Triamidotriphenyl  carbinol,  the  pararosaniline  colour-base, 
is  obtained,  as  a  colourless  precipitate,  on  adding  alkalies  to  a 
solution  of  the  chloride,  or  of  some  other  salt  ;  it  crystallises 
from  alcohol  in  colourless  needles,  and,  when  treated  with 
acids,  gives  the  intensely  coloured  pararosaniline  salts. 

Leuco-pararosaniline,  paraleucaniline  or  triamidotriphenyl- 
methane,  NH2-C6H4-CH(C6H4-NH2)2,  is  prepared  by  reducing 
triamidotriphenyl  carbinol  with  zinc-dust  and  hydrochloric  acid, 

NH2.C6H4.C(OH)(C6H4.NH2)2  +  2H  = 

NH2.C6H4.CH(C6H4.NH2)2  +  H20. 

It  crystallises  in  colourless  plates,  melts  at  148°,  and  forms 
salts,  such  as  the  hydrochloride,  Cl9H.igNB,3~H.C\,  with  three 
equivalents  of  an  acid.  When  the  hydrochloride  is  treated 
with  nitrous  acid,  it  is  converted  into  a  tri-diazo-compound, 
CH(C6H4'N:NC1)3,  which,  when  boiled  with  water,  yields 


DYES    AND    THEIR    APPLICATION.  513 

aurin,  C19HU03  (p.  518),  and  when  heated  with  alcohol,  is 
converted  into  triphenylmethane,  just  as  diazobenzene  chloride, 
under  similar  conditions,  yields  phenol  or  benzene. 

Constitution  of  Pararosaniline.  —  Since  triphenylmethane  can 
be  obtained  from  pararosaniline  in  this  way,  the  latter  is  a 
derivative  of  this  hydrocarbon  (an  important  fact,  first  estab- 
lished by  E.  and  0.  Fischer  in  1878)  ;  moreover,  pararosani- 
line may  be  prepared  from  triphenylmethane,  as  follows  : 
Triphenylmethane  is  converted  into  trinitrotriphenylmethane, 
;N"02-C6H4-CH(C6H4.j^02)2—  a  compound  in  which,  it  has. 
been  shown,  that  all  the  nitro-groups  are  in  the  ^-position  to 
the  methane  carbon  atom*  —  with  the  aid  of  fuming  nitric  acid  > 
this  rlifro-compound,  on  reduction,  yields  a  substance  which  is 
identical  with  leuco-pararosaniline,  and  which,  on  oxidation, 
is  readily  converted  into  the  colour-base,  triamidotriphenyl 
carbinol  ;  this  base,  when  treated  with  acids,  yields  salts  of 
pararosaniline,  with  elimination  of  water  (compare  p.  511)  : 


Hydrochloride  of  Pararosaniline 
Base 


Chloride  of  Pararosaniline. 

Rosaniline  (of  commerce),  fuchsine,  or  magenta,  is  the 
chloride  (or  acetate)  of  triamidotolyldiphenyl  carbinol,  a  base 
which  is  produced  by  the  oxidation  of  equal  molecular  pro- 
portions of  aniline,  o-toluidine,  and  ^-toluidine  (with  arsenic 
acid,  mercuric  nitrate,  nitrobenzene,  &c.),  the  reaction  being 
similar  in  all  respects  to  the  formation  of  the  pararosaniline 
base  from  aniline  (2  mols.)  and  ^>-toluidine  (1  mol.), 

o-Toluidine. 


NH2.C6H4.CH3  4-  £6£  XH         -2  +  30  = 

p-Toluidine.  Aniline. 


Rosaniline  Base. 

*  The  proofs  of  this  statement  are  too  complex  to  be  given  here. 
2G 


514  DYES   AND   THEIR   APPLICATION. 

Rosaniline  is  usually  manufactured  at  the  present  time  by 
what  is  termed  the  '  nitrobenzene  process?  the  '  arsenic  acid 
process ' — in  which  the  oxidising  agent  is  arsenic  acid — heing 
now  little  used. 

To  the  requisite  mixture  of  aniline,  o-toluidine,  and  jt>-toluidine* 
(38  parts),  hydrochloric  acid  (20  parts)  and  nitrobenzene  (20  parts)  are 
added,  and  the  whole  is  gradually  heated  to  190°,  small  quantities 
of  iron-filings  (3-5  parts)  being  added  from  time  to  time  (see  below). 
At  the  end  of  five  hours  the  reaction  is  complete,  and  steam  is  then 
led  through  the  mass  to  drive  off  any  unchanged  aniline,  toluidine, 
or  nitrobenzene,  after  which  the  residue  is  powdered  and  extracted 
with  boiling  water,  under  pressure  ;  lastly,  the  extract  is  mixed 
with  salt,  and  the  crude  rosaniline  chloride  which  separates  purified 
by  recrystallisation. 

In  this  reaction  the  nitrobenzene  acts  only  indirectly  as  the 
oxidising  agent ;  the  ferrous  chloride,  produced  by  the  action  of  the 
hydrochloric  acid  on  the  iron,  is  oxidised  by  the  nitrobenzene  to 
ferric  chloride,  which  in  its  turn  oxidises  the  mixture  of  aniline 
and  toluidiues  to  rosaniline,  and  is  itself  again  reduced  to  ferrous 
chloride ;  the  action  is,  therefore,  continuous,  and  only  a  small 
quantity  of  iron  is  necessary. 

The  salts  of  the  rosaniline  base  with  one  equivalent  of  acid, 
as,  for  example,  the  chloride,  C20H20N3C1,  form  magnificent 
crystals,  which  show  an  intense  green  metallic  lustre;  they 
dissolve  in  warm  water,  forming  deep  red  solutions,  and  dye 
silk,  wool,  and  cotton  a  brilliant  magenta  colour,  the  con- 
ditions of  dyeing  being  the  same  as  in  the  case  of  malachite 
green. 

The  addition  of  alkalies  to  the  saturated  solution  of  the 
chloride  of  rosaniline  destroys  the  colour,  and  causes 
the  precipitation  of  the  colour-base,  triamidotolyldiplienyl 
carbinol,  C20H20N3-OH  (p.  511),  which  crystallises  in  colour- 
less needles,  and,  on  warming  with  acids,  is  at  once  reconverted 
into  the  intensely  coloured  salts.  When  reduced  with  tin 
and  hydrochloric  acid,  the  rosaniline  salts  yield  leuco-ros- 
aniline,  C20H21N3  (p.  511),  a  colourless,  crystalline  substance, 

*  Crude  'aniline-oil,'  a  mixture  of  these  three  bases,  is  sometimes  used 
instead  of  the  pure  compounds. 


DYES   AND   THEIR   APPLICATION.  515 

which,  when  treated  with  oxidising  agents,  is  again  converted 
into  rosaniline. 

The  constitution  of  rosaniline  has  been  deduced  in  the  same 
way  as  that  of  pararosaniline  (p.  513),  since,  by  means  of  the 
diazo-reaction,  leuco-rosaniline  has  been  converted  into 
diphenyl-??i-tolylmethane,  CH3-C6H4-CH(C6H5)2 ;  leuco-rosani- 
line has,  therefore,  the  constitution 

(4) 
(4)' 

and  the  rosaniline  salts  are  derived  from  this  base,  just  as 
those  of  pararosaniline  and  of  malachite  green  are  derived  from 
leuco -pararosaniline  and  leuco-malachite  green  respectively. 

Derivatives  of  Pararosaniline  and  Rosaniline. 

The  hydrogen  atoms  of  the  three  amido-groups  in  pararos- 
aniline and  rosaniline  may  be  displaced  by  methyl-  or  ethyl- 
groups,  by  heating  the  dye  with  methyl  or  ethyl  iodide 
(chloride  or  bromide) ;  under  these  conditions,  tri-alkyl  sub- 
stitution products  are  obtained  as  primary  products,  one  of 
the  hydrogen  atoms  of  each  of  the  amido-groups  being  dis- 
placed. When,  for  example,  rosaniline  chloride  is  heated 
with  methyl  iodide  or  chloride,  it  yields,  in  the  first  place,  the 
chloride  of  trimethyl-Tosamlme, 


This  compound  is  a  reddish-violet  dye ;  the  corresponding 
trietJnjl-i-osamlme  chloride  is  the  principal  constituent  of  Hof- 
mann's  violet,  dahlia,  primula,  &c.  dyes,  which  have  now 
been  superseded  by  more  brilliant  violets. 

By  the  long-continued  action  of  the  methyl  halogen  com- 
pounds on  rosaniline  salts,  the  chloride  of  hexamethyl-Tosani- 
line, 


516  DYES   AND   THEIR   APPLICATION. 

is  obtained.  This  substance  is  a  magnificent,  bluish-violet 
dye,  but  is  now  little  used;  it  is  a  tertiary  base,  and,  like 
dimethylaniline,  it  combines  directly  with  methyl  chloride, 
forming  an  additive  compound  of  the  constitution 


which,  curiously  enough,  is  green,  and  was  formerly  used 
under  the  name  '  iodine  green  '  (so  called  because  it  was  first 
produced  with  methyl  iodide). 

Starting,  then,  from  rosaniline,  which  is  a  brilliant  red  dye, 
and  substituting  methyl-groups  for  hydrogen,  the  colour  first 
becomes  reddish-violet,  and  then  bluish-violet,  as  the  number 
of  alkyl-groups  increases.  This  change  is  more  marked  when 
ethyl-groups  are  introduced,  and,  still  more  so,  when  phenyl- 
or  benzyl-groups  are  substituted  for  hydrogen,  as,  in  the  latter 
case,  pure  blue  dyes  are  produced  (see  below)  ;  in  fact,  by 
varying  the  number  and  character  of  the  substituting  groups, 
almost  any  shade  from  red  to  blue  can  be  obtained. 

Lastly,  it  is  interesting  to  note  that,  when  a  violet  dye,  like 
hexamethylrosaniline,  combines  with  an  alkyl  halogen  com- 
pound, it  is  converted  into  a  bright  green  dye,  which,  how- 
ever, is  somewhat  unstable,  and,  on  warming,  readily  decom- 
poses into  the  alkyl  halogen  compound  and  the  original  violet 
dye.  A  piece  of  paper,  for  example,  which  has  been  dyed 
with  *  iodine  green  '  becomes  violet  when  warmed  over  a 
bunsen  burner,  and  methyl  chloride  is  evolved. 

The  alkyl-derivatives  of  pararosaniline  and  of  rosaniline  are 
no  longer  prepared  by  heating  the  dyes  with  alkyl  halogen 
compounds,  but  are  obtained  by  more  economical  methods. 
The  dyes  of  this  class  now  actually  manufactured,  examples 
of  which  are  described  below,  are,  with  few  exceptions,  deriv- 
atives of  pararosaniline. 

Methyl  violet  appears  to  consist  principally  of  the  chloride 
of  j9erctarae£%/-pararosaniline  ;  it  is  usually  manufactured  by 
heating  a  mixture  of  dimethylaniline,  potassium  chlorate, 


DYES   AND   THEIR   APPLICATION.  517 

and  copper  chloride  (or  sulphate),  at  50-60°,  for  about  8 
hours;*  the  product  is  treated  with  hot  water,  the  copper 
removed  by  passing  sulphuretted  hydrogen,  the  solution 
concentrated,  and  the  dye  precipitated  by  the  addition  of 
salt. 

Methylviolet  comes  into  the  market  in  the  form  of  hard 
lumps,  which  have  a  green  metallic  lustre ;  it  is  readily  soluble 
in  alcohol  and  hot  water,  forming  beautiful  violet  solutions, 
which  dye  silk,  wool,  and  cotton,  under  the  same  conditions 
as  employed  in  the  case  of  malachite  green  (p.  511). 

When  rosaniline  is  treated  with  aniline  at  100°,  in  the 
presence  of  some  weak  acid,  such  as  acetic,  benzoic,  or  stearic 
acid  (which  combines  with  the  ammonia),  phenyl-groups  dis- 
place the  hydrogen  atoms  of  the  amido-groups,  just  as  in  the 
formation  of  diphenylamine  from  aniline  and  aniline  hydro- 
chloride  (p.  368), 

C6H6.NH2  +  C6H5.NH2,HC1  =  (C6H5)2NH  +  NH3,HC1. 

Here,  as  in  the  case  of  the  alkyl-derivatives  of  rosaniline,  the 
colour  of  the  product  depends  on  the  number  of  phenyl-groups 
which  have  been  introduced;  the  mono-  and  di-phenyl- 
derivatives  are  reddish-violet  and  bluish-violet  respectively, 
whereas  triphenylrosaniline  is  a  pure  blue  dye,  known  as 
aniline  blue. 

Aniline  blue,  C 

(triphenylrosaniline  chloride),  is  prepared  by  heating  rosaniline 
with  benzoic  acid  and  an  excess  of  aniline  at  180°  for  about 
4  hours,  and  until  the  mass  dissolves  in  dilute  acids,  forming 
a  pure  blue  solution.  The  product,  which  contains  the  aniline 
blue  in  the  form  of  the  colour-base,  is  then  treated  with 
hydrochloric  acid,  whereupon  the  chloride  crystallises  out  in 
an  almost  pure  condition. 

*  The  changes  which  take  place  during  this  remarkable  process  are 
doubtless  very  complex,  and  cannot  be  discussed  here* 


518  DYES   AND   THEIR   APPLICATION. 

Aniline  blue  is  very  sparingly  soluble  in  water,  and,  in 
dyeing  with  it,  the  operation  has  to  be  conducted  in  alcoholic 
solution.  In  order  to  get  over  this  difficulty,  the  insoluble 
dye  is  treated  with  anhydrosulphuric  acid,  and  thus  converted 
into  a  mixture  of  sulphonic  acids,  the  sodium  salts  of  which 
are  readily  soluble,  and  come  into  the  market  under  the 
names  '  alkali  blue,'  t  water  UueJ  &c. 

In  dyeing  silk  and  wool  with  these  colouring  matters,  the 
material  is  first  dipped  into  alkaline  solutions  of  the  salts, 
when  a  light-blue  tint  is  obtained,  and  it  is  not  until  it  has 
been  immersed  in  dilute  acid  (to  liberate  the  sulphonic  acid), 
that  the  true  blue  colour  is  developed.  Cotton  is  dyed  in 
the  same  way,  but  must  first  be  mordanted  with  tannin. 

The  tri-hydroxy-derivatives  of  triphenyl  carbinol  and  of  tolyldi- 
phenyl  carbinol,  which  correspond  with  the  tri-amido-com  pounds 
described  above,  are  respectively  represented  by  the  following 
formulae  : 


Trihydroxytriphenyl  Carbinol.  Trihydroxytolyldiphenyl  Carbinol. 

These  compounds  may  be  obtained  from  the  corresponding  tri- 
amido-derivatives  (the  colour-bases  of  pararosaniline  and  of  rosani- 
line)  with  the  aid  of  the  diazo-reaction  ;  in  other  words,  the  amido- 
compounds  are  treated  with  nitrous  acid,  and  the  solutions  of  the 
diazo-salts  are  then  heated.  The  hydroxy-compounds  thus  produced 
are,  however,  unstable,  and  readily  lose  one  molecule  of  water, 
yielding  coloured  compounds—  aurin  and  rosolic  acid—  which  corre- 
spond with  the  pararosaniline  and  rosaniline  dyes  in  constitution, 


Aurin.  Rosolic  Acid. 

These  substances  are  of  little  use  as  dyes  owing  to  the  difficulty  of 
fixing  them. 

The  Plitlialeins. 

The  phthaleins,  like  malachite  green  and  the  rosanilines, 
are  derivatives  of  triphenylmethane,  inasmuch  as  they  are 
substitution  products  of  phthalophenone,  a  compound  formed 


DYRS    AND    THEIR    APPLICATION.  519 

from  triphenylcarbinol-o-carboxylic  acid,  by  loss  of  one  mole- 
cule of  water,* 

CO<OH  $b>°(C6H5>2  =  CO<^^>C(C6H5)2  +  H20. 
Phthalophenone  is  readily  prepared  by  acting  on  a  mixture 
of  phthalyl  chloride  (p.  426)  and  benzene,  with  aluminium 
crhloride, 

2CH  =  C<KCCH      +  2HC1. 


It  crystallises  in  colourless  needles,  melts  at  115°,  and 
dissolves  in  alkalies,  yielding  salts  of  triphenylcarbinol-o-carb- 
oxylic  acid.  This  acid,  on  reduction  with  zinc-dust  in 
alkaline  solution,  is  converted  into  triphenylmethane-o-carb- 
oxylic  add,  COOH-C6H4-CH(C6H5),,  from  which,  by  distilla- 
tion with  lime,  triirfienylmethane  is  obtained  —  a  proof  that 
the  phthaleins  are  derivatives  of  this  compound. 

Phenolphthalem,  or  dihydroxyphthalophenone,  C20H1404,  is 
prepared  by  heating  phthalic  anhydride  (3  parts)  with  phenol 
(4  parts)  and  powdered  zinc  chloride  (5  parts),  at  115-120° 
for  8  hours  ;  the  product  is  washed  with  water,  dissolved  in 
soda,  and  the  phenolphthalem  precipitated  from  the  filtered 
solution  with  acetic  acid, 

2C6H5-OH  = 


H20. 

*  Compounds  produced  in  this  way  from  one  molecule  of  a  hydroxy- 
acid,  by  loss  of  water,  are  called  lactones.  Many  hydroxy-acids,  notably 
those  belonging  to  the  fatty  series,  yield  lactones,  but  only  when  the 
hydroxyl-group  is  in  the  7-  or  5-position  (part  i.  p.  164). 

7-Hydroxybutyric  acid,  for  example,  cannot  be  isolated,  because  when 
set  free  from  its  salts,  by  the  addition  of  a  mineral  acid,  it  at  once  decom- 
poses with  formation  of  its  lactone, 

CH2(OH)  CH2-CH2  COOH  =  CH2'CH2-CH2+  H2O. 

»y-Butyrolactone. 

The  fatty  lactones  are  mostly  neutral  volatile  liquids,  but  those  belonging  to 
the  aromatic  series  are  crystalline  solids  ;  all  lactones  dissolve  in  alkalies, 
yielding  salts  of  the  hydroxy-acids  from  which  they  are  derived. 


520  DYES    AND    THEIR    APPLICATION. 

Phenolphthalem  separates  from  alcohol  in  small  yellowish 
crystals,  and  melts  at  250°  ;  its  solutions  are  coloured  a  deep 
pink  .  on  the  addition  of  alkali,  owing  to  the  formation  of  a 
salt,  but  the  colour  is  destroyed  by  acids,  hence  the  use  of 
phenolphthalem  as  an  indicator  in  alkalimetry;  it  is,  however, 
of  no  value  as  a  dye. 

That  phenolphthalei'n  is  dihydroxyphthalophenone,  and,  there- 
fore, a  derivative  of  triphenylmethane,  may  be  proved  in  the 
following  way.  Phthalophenone,  when  treated  with  nitric  acid, 
yields  dinitrophthalophenone,  which,  on  reduction,  is  converted 
into  diamidophthalophenone  :  from  this  substance,  by  treatment 
with  nitrous  acid,  phenolphthalei'n  is  produced. 

CO<S6oIf>C(C6H4-N02)2  CO<^t>C(C6 

Dinitrophthalophenone.  Diamidophthalophenone. 


Phenolphthalem. 

Fluorescein,  C20H1205,  is  a  very  important  dye-stuff, 
produced  by  heating  together  phthalic  anhydride  and  resor- 
cinol, 


Fluorescein. 

In  this  change,  two  hydrogen  atoms  of  the  two  benzene  rings 
unite  with  the  oxygen  atom  of  one  of  the  >CO  groups  of  the 
phthalic  anhydride  (as  in  the  formation  of  phenolphthalei'n), 
a  second  molecule  of  water  being  eliminated  from  the  hydroxyl- 
groups  of  the  two  resorcinol  molecules. 

Phthalic  anhydride  (5  parts)  and  resorcinol  (7  parts)  are  heated 
together  at  200°  until  the  mass  has  become  quite  solid  ;  the  dark 
product  is  then  washed  with  hot  water,  dissolved  in  soda,  the  filtered 
alkaline  solution  acidified  with  sulphuric  acid,  and  the  fluorescein 
extracted  with  ether. 

Fluorescein  crystallises  from  alcohol  in  dark-red  crusts  ;  it 
is  almost  insoluble  in  water,  but  dissolves  readily  in  alkalies, 


DYES    AND    THEIR   APPLICATION.  521 

forming  dark  reddish-brown  solutions,  which,  when  diluted, 
show  a  most  magnificent  yellowish-green  fluorescence  (hence 
the  name  fluorescein).  In  the  form  of  its  sodium  salt, 
C20H1005Xa2,  fluorescein  comes  into  the  market  as  the  dye 
'  uranin.'  Woor  and  silk  are  dyed  yellow,  and  at  the  same 
time  show  a  beautiful  fluorescence,  but  the  colours  are  faint, 
and  soon  fade,  hence  fluorescein  has  a  very  limited  application 
alone,  and  is  generally  mixed  with  other  dyes,  in  order  to 
impart  fluorescence.  The  great  value  of  fluorescein  lies  in  the 
fact  that  its  derivatives  are  very  important  dyes. 


Eosin,  CKcroE0  (tetrabromonubr- 
escein),  is  formed  when  fluorescein  is  treated  with  bromine,  four 
atoms  of  hydrogen  in  the  resorcinol  nuclei  being  displaced. 

Flnorescei'n  is  treated  with  the  calculated  quantity  of  bromine 
in  acetic  acid  solution,  and  the  eosin  which  separates  is  collected, 
washed  with  a  little  acetic  acid,  and  dissolved  in  dilute  potash.  The 
filtered  solution  is  then  acidified,  and  the  eosin  extracted  with 
ether. 

Eosin  separates  from  alcohol  in  red  crystals,  and  is  almost 
insoluble  in  water,  but  dissolves  readily  in  alkalies,  forming 
deep-red  solutions,  which,  on  dilution,  exhibit  a  beautiful 
green  fluorescence,  but  not  nearly  to  the  same  extent  as 
solutions  of  fluorescein. 

Eosin  comes  into  the  market  in  the  form  of  its  potassium  salt, 
C20H6Br405K2  (a  brownish  powder),  and  is  much  used  for 
dyeing  silk,  wool,  cotton,  and  especially  paper,  which  fixes 
the  dye  without  the  aid  of  a  mordant.  Silk  and  wool  are 
dyed  with  eosin  directly  in  a  bath  acidified  with  a  little 
acetic  acid  ;  but  cotton  must  first  be  mordanted  with  zinc, 
lead,  or  aluminium  salts.  The  shades  produced  are  a  beautiful 
pink,  and  the  materials  also  show  a  very  beautiful  fluorescence. 

Tetriodofluoresce'in,  C20H8I405,  is  also  a  valuable  dye. 
Its  sodium  salt,  C20H6I405Na2,  comes  into  the  market  under 
the  name  'erythroMii' 

Many  other  phthaleins  have  been  prepared  by  condensing 


522  DYES   AND    THEIR    APPLICATION. 

phthalic  acid  and   its  derivatives  with    other   phenols,  and 
then  treating  the  products  with  bromine  or  iodine. 

Azo-dyes. 

The  azo-dyes  contain  the  azo-group,  -N:N-,  to  each  of  the 
nitrogen  atoms  of  which  a  benzene  or  naphthalene  nucleus  is 
directly  united.  Azobenzene,  C6H5-N:N-C6H5,  the  simplest 
of  all  azo-compounds,  is  not  a  dye,  although  it  is  intensely 
coloured  (compare  p.  502),  and  this  is  true  also  of  other 
neutral  azo-cornpounds  ;  if,  however,  one  or  more  hydrogen 
atoms  in  such  compounds  be  displaced  by  amido-,  hydroxyl-,  or 
sulphonic-groups,  the  products,  as,  for  example, 

Amidoazobenzene,  C6H5-N:N.C6H4.:NTH2, 

Hydroxy  azobenzene,  C6H5-  -^  :-^'  ^6^4  *  OH, 

Azobenzenesulphonic  acid,  C6H5-N:N-CGH4-S03H, 

are  yellow  or  brown  dyes. 

Azo-dyes    are    usually  prepared   by   one    of    two    general 
methods  —  namely,  by  treating  a  diazo-chloride  with  an  amido- 
compound* 
C6H5'N*TC1  +  C6H6.N(CH3)2  =  C6H5.N:N.C6H4.N(CH3)2,HC1, 

Dimethylamidoazobenzene  Hydrochloride. 

CH3.C6H4.N:NC1  +  CH3.C6H4-NH2  = 

p-Diazotoluene  Chloride.  o-Toluidine. 


Amidoazotoluen  e  Hydrochloride. 

or  by  treating  a  diazo-chloride  with  a,  phenol, 

+  C6H5.OH  =  C6H5.N:N.C6H4.OH  +  HC1, 

Hydroxyazobeiizene. 

C6H4(OH)2  =  C6H5.N:N-C6H3(OH)2  +  HC1. 

Dihydroxyazobenzeue. 

In  the  first  case  the  products  —  amidoazo-compounds  —  are 
basic  dyes,  whereas  in  the  second  case  they  are  acid  dyes. 

*  In  cases  where  a  diazoamido-compound  is  first  produced  (p.  374),  an 
excess  of  the  amido-compound  is  employed  and  the  mixture  warmed  until 
the  intramolecular  change  into  the  amidoazo-compound  is  complete. 


DYES   AND   THEIR   APPLICATION.  523 

Another  method  of  some  general  application  for  the  direct 
preparation  of  azo-dyes  containing  a  sulphonic-group,  consists 
in  treating  diazobenzenesulphonic  acid,  or  its  anhydride  (p. 
384),  with  an  amido-compound  or  with  a  phenol : 

S03H-C6H4.N:N-OH  +  C6H5-NH2  = 

S03H.C6H4.N:N-C6H4.]STH2  +  H20 

Amidoazobenzenesulphonic  Acid. 

S03H.C6H4.N:N.OH  +  C6H5-OH  = 

S03H-C6H4.N:N.C6H4.OH  +  H20. 

Hydroxyazobenzenesulphouic  Acid. 

As,  however,  the  yield  is  generally  a  poor  one,  such  dyes  are 
usually  prepared  by  sulphonating  the  amidoazo-  or  hydroxy- 
azo-com  pounds. 

In  all  these  reactions  the  diazo-group,  C6H5'N:£T-,  displaces 
hydrogen  of  the  benzene  nucleus  from  the  ^-position  to  one  of 
the  amido-  or  hydroxyl-groups  ;  substances  such  as  jo-toluidiue, 
in  which  the  ^-position  is  occupied,  either  do  not  interact 
with  diazo-chlorides  or  only  do  so  with  great  difficulty. 

The  technical  operations  incurred  in  the  production  of  azo-colours 
are,  as  a  rule,  very  simple.  In  combining  diazo-compounds  with 
phenols,  for  example,  the  amido-compound  (1  mol.)  is  dissolved  in 
water  and  hydrochloric  acid  (2  mols. ),  the  solution  well  cooled  with 
ice,  and  gradually  mixed  with  the  calculated  quantity  of  sodium 
nitrite  (1  mol.);  this  solution  of  the  diazo-salt  is  then  slowly  run 
into  the  alkaline  solution  of  the  phenol,  or  its  sulphonic  acid,  care 
being  taken  to  keep  the  solution  slightly  alkaline,  otherwise  the 
liberated  hydrochloric  acid  prevents  combination  taking  place. 
After  a  short  time  the  solution  is  mixed  with  salt,  which  causes  the 
colouring  matter  to  separate  in  flocculent  masses  ;  the  product  is 
then  collected  in  filter-presses  and  dried,  or  sent  into  the  market  in 
the  form  of  a  paste. 

The  combination  of  diazo-compounds  with  amido-compounds  is 
generally  brought  about  by  simply  mixing  the  aqueous  solution  of 
the  diazo-compound  with  that  of  the  salt  of  the  amido-compound 
(compare  foot-note,  p.  522),  and  then  precipitating  the  colouring 
matter  by  the  addition  of  common  salt ;  in  some  cases,  however, 
the  reaction  takes  place  only  in  alcoholic  solution. 

Acid  azo-colours  (that  is,  hydroxy-  and  sulphonic-derivatives) 


524  DYES    AND    THEIR    APPLICATION. 

are  taken  up  by  animal  fibres  directly  from  an  acid  bath,  and 
are  principally  employed  in  dyeing  wool  ;  they  can  be  fixed 
on  cotton  with  the  aid  of  mordants  (tin  and  aluminium  salts 
being  generally  employed),  but,  as  a  rule,  only  with  difficulty  ; 
nevertheless  some  acid  dyes,  notably  those  of  the  Congo-group 
(p.  526),  dye  cotton  directly  without  a  mordant. 

Basic  azo-dyes  are  readily  fixed  on  cotton  which  has  been 
mordanted  with  tannin,  and  are  very  largely  used  in  dyeing 
calico  and  other  cotton  goods. 

At  the  present  time  a  great  many  azo-colours  are  manu- 
factured, but  only  a  few  of  the  more  typical  can  be  mentioned 
here. 

Aniline  yellow,    a    salt    of    amidoazobenzene    (p.    375), 


is  now  no  longer  used  in  dyeing,  because  the  colour  is  not 
fast,  and  is  in  many  ways  inferior  to  other  readily  obtainable 
yellow  dyes. 

Chrysoidine  (diamidoazobenzene),  C6H5-N:N.C6H3(NH2)2, 
is  produced  by  mixing  molecular  proportions  of  diazobenzene 
chloride  and  m-phenylenediamine  (p.  364)  in  aqueous  solution. 
The  hydrochloride  crystallises  in  reddish  needles,  is  moderately 
soluble  in  water,  and  dyes  silk  and  wool  directly,  and  cotton 
mordanted  with  tannin,  an  orange-yellow  colour. 

Bismarck  brown,  NH2.C6H4-N:IsT.C6H3(NH2)2  (triamidoazo- 
benzene),  is  prepared  by  treating  m-phenylenediamine  hydro- 
chloride  with  nitrous  acid,  one  half  of  the  base  being  con- 
verted into  the  diazo-compound,  which  then  interacts  with 
the  other  half,  producing  the  dye, 

NH2.C6H4.N:NC1  +  C6H4(NH2)2  - 

NH2.C6H4.N:N.C6H3(NH2)2,HC1. 

The  hydrochloride  is  a  dark-brown  powder,  and  is  largely  used 
in  dyeing  cotton  (mordanted)  and  leather  a  dark  brown. 

Helianthin  (dimethylaraidoozohenzenesulphonic  acid)  is 
very  easily  prepared  by  mixing  aqueous  solutions  of 


DYES    AND    THEIR   APPLICATION.  525 

diazobenzenesulphonic  acid  and  dimethylaniline  hydro- 
chloride, 

S03ILC6H4.N:N.OH  +  C6H5-N(CH3)2  = 

S03H.C6H4.IST:N.C6H4.N(CH3)2  +  H20. 

The  sodium  salt  (methylorange)  is  a  brilliant  orange-yellow 
powder,  and  dissolves  freely  in  hot  water,  forming  a  yellow 
solution,  which  is  coloured  red  on  the  addition  of  acids,  hence 
its  use  as  an  indicator.  It  is  seldom  employed  as  a  dye,  on 
account  of  its  sensibility  to  traces  of  acid. 

Resorcin  yellow  (tropseolin  0)  is  prepared  by  combining 
diazobenzenesulphonic  acid  and  resorcinol,  and  has  the  con- 
stitution S03H.C6H4-]Sr:]S~.C6H3(OH)2.  Its  sodium  salt  is  a 
moderately  brilliant  orange-yellow  dye,  and  is  not  readily 
acted  on  by  acids ;  it  is  chiefly  employed,  mixed  with  other 
dyes  of  similar  constitution,  in  the  production  of  olive- 
greens,  maroons,  &c. 

By  using  various  benzene  derivatives,  and  combining  them 
as  in  the  above  examples,  yellow  and  brown  dyes  of  almost 
any  desired  shade  can  be  obtained ;  in  order,  however,  to 
produce  a  red  azo-dye,  a  compound,  containing  at  least  one 
naphthalene  nucleus,  must  be  prepared.  This  can  be  readily 
done  by  combining  a  benzenediazo-compound  with  a  naphthyl- 
amine,  naphthol,  naphthalenesulphonic  acid,  &c.,  just  as 
described  above.  The  dyes  thus  obtained  give  various 
shades  of  reddish-brown  or  scarlet,  and  are  known  collectively 
as  '  Ponceaux'  or  *  Bordeaux.' 

When,  for  example,  diazoxylene  chloride  is  combined  with 
/?-naphthol,  a  scarlet  dye  (scarlet  K)  of  the  composition 
C6H3(CH3)2.N:N.C10H5(OH).S03Na  is  formed;  another  scarlet 
dye  (Ponceau  3R)  is  produced  by  the  combination  of  diazo- 
cumene  chloride  with  /3-naphtholdisulphonic  acid,  and  has 
the  composition  C6H2(CH3)8.N:N.C10H4(S08Na)2.()H. 

Rocellin,  S03Na.C10H6.X:N.C10H6-OH,  a  compound  pro- 
duced by  combining  ^-naphthol  with  the  diazo-compound  of 
naphthionic  acid  (p.  455),  may  be  mentioned  as  an  example 


DYES    AND    THEIR   APPLICATION. 

of  an  azo-dye  containing  two  naphthalene  nuclei.  It  gives 
beautiful  red  shades,  very  similar  to  those  obtained  with  the 
natural  dye,  cochineal,  which  rocellin  and  other  allied  azo- 
colours  have,  in  fact,  almost  superseded. 

Within  the  last  few  years  a  great  number  of  exceedingly 
valuable  colouring  matters  have  been  prepared  from  benzidine, 
NH2.C6H4.C6H4.NH2  (p.  379),  and  its  derivatives. 

Benzidine  may  be  compared  with  two  molecules  of  aniline, 
and  when  diazotised  it  yields  the  salt  of  a  di-diazo-  or  tetrazo- 
dipkenyl,  C1N:^C6H4.C6H4.N:NC1.  This  substance  inter- 
acts with  amido-com  pounds,  phenols,  and  their  sulphonic  acids, 
just  as  does  diazobenzene  chloride  (but  with  double  the 
quantity),  producing  a  variety  of  most  important  colouring 
matters,  known  as  the  dyes  of  the  conga-group. 

Congo-red,  a  dye  produced  by  the  combination  of  tetrazo- 
diphenyl  chloride  with  naphthionic  acid,  is  one  of  the  most 
valuable  compounds  of  this  class.  Its  sodium  salt, 


is  a  scarlet  powder,  which,  on  the  addition  of  acids,  turns 
blue,  owing  to  the  liberation  of  the  free  sulphonic  acid. 

The  congo-dyes  possess  the  unusual  property  of  com- 
bining with  unmordanted  cotton,  producing  brownish-red 
shades  which  are  fast  to  soap.  They  are  much  used  for  dye- 
ing cotton,  but  they  become  dull  in  time  in  any  atmosphere 
which  contains  traces  of  acid  fumes,  as,  for  example,  in  the 
air  of  manufacturing  towns,  owing  to  the  liberation  of  the 
blue  sulphonic  acids. 

The  Benzopurpurins  are  also  exceedingly  valuable  dyes  of 
the  congo-group  ;  they  are  produced  by  combining  tetrazo- 
ditolyl  salts*  with  the  sulphonic  acids  of  a-  and  /3-naphthyl- 
amine,  and  are,  therefore,  very  similar  to  congo-red  in  con- 

*Tolidine,  NH2-(CH3)C6H8-C6H3(CH3)-NH2,  is  produced  from  nitro- 
toluene  by  reactions  similar  to  those  by  which  benzidine  is  produced  from 
nitrobenzene  ;  when  its  salts  are  treated  with  nitrous  acid  they  yield  salts 
of  tetrazoditolyl,  just  as  benzidine  gives  salts  of  tetrazodiphenyl, 


DYES    AND    THEIR    APPLICATION.  527 

stitution.     They  dye    unmordanted  cotton    splendid   scarlet 
shades,  and  are  used  in  very  large  quantities. 


Various  Colouring  Matters. 

Martins'  yellow  (dhiitro-a-naphthol),  C10H5(N02)2-OH,  is 
obtained  by  the  action  of  nitric  acid  on  a-naplitliolniono-,  or 
di-sulphonic  acid,  the  sulphonic  group  or  groups  being  elimin- 
ated during  nitration.  The  commercial  dye  is  the  sodium 
salt,  C10H5(N02)2-ONa ;  it  is  readily  soluble  in  water,  and 
dyes  silk  and  wool  directly  an  intense  golden  yellow. 

When  a-naphthol-trisulphonic  acid  is  nitrated,  only  two  of 
the  sulphonic  groups  are  eliminated,  and  the  resulting  sub- 
stance has  the  formula  C10H4(N02)2(OH).S03H;  it  is,  in  fact, 
the  sulphonic  acid  of  Martins'  yellow.  This  valuable  dye-stuff 
is  called  naphthol  yellow,  and  comes  into  the  market  in  the 
form  of  its  potassium  salt,  C10H4(M)2)2(OH)-S03K;  it  is 
very  largely  used,  as  the  yellow  shades  are  faster  to  light 
than  those  of  Martins'  yellow. 

Methylene  blue,  C16H18N3SC1,  was  first  prepared  by  Caro, 
in  1876,  by  the  oxidation  of  dimethyl-^-phenylenediamine 
(p.  367)  with  ferric  chloride  in  presence  of  sulphuretted 
hydrogen. 

Nitrosodimethylaniline  (p.  367)  is  reduced  in  strongly  acid 
solution  with  zinc-dust,  or  with  sulphuretted  hydrogen,  and  the 
solution  of  dimethyl -p-phenylenediamine  thus  obtained  is  treated 
with  ferric  chloride  in  presence  of  excess  of  sulphuretted  hydrogen. 
The  intensely  blue  solution  thus  obtained  is  mixed  with  salt  and 
zinc  chloride,  which  precipitate  the  colouring  matter  as  a  zinc 
double  salt,  in  which  form  it  comes  into  the  market. 

Methylene  blue  is  readily  soluble  in  water,  and  is  a  valuable 
cotton-blue,  as  it  dyes  cotton,  mordanted  with  tannin,  a 
beautiful  blue,  which  is  very  fast  to  light  and  soap;  it  is 
not  much  used  in  dyeing  silk  or  wool. 

Indigo,  C]6H10N202,  is  a  natural  dye,  which  has  been  used 
from  the  earliest  times.  It  is  contained  in  the  leaves  of  the 
indigo  plant  (Indigo/era  tinctoria)  and  in  woad  (Isatis  tindoria) 


528  DYES    AND    THEIR    APPLICATION. 

in  the  form  of  the  glucoside  *  indican ; '  when  the  leaves  are 
macerated  with  water,  this  glucoside  undergoes  fermentation, 
and  indigo  separates  as  a  blue  scum. 

Indigo  comes  into  the  market  in  an  impure  condition  in 
the  form  of  dark-blue  lumps,  and,  especially  when  rubbed, 
shows  a  remarkable  copper-like  lustre  \  it  is  insoluble  in 
water  and  most  other  solvents,  but  dissolves  readily  in  hot 
aniline,  from  which  it  crystallises  on  cooling ;  it  sublimes, 
when  heated,  in  the  form  of  a  purple  vapour,  and  condenses 
as  a  dark-blue  crystalline  powder,  which  consists  of  pure 
'indigotin,'  the  principal  and  most  valuable  constituent  of 
commercial  indigo. 

Reducing  agents  convert  indigo  into  its  leuco-compound, 
indigo  white^  which,  in  contact  with  air,  is  rapidly  recon- 
verted into  indigo,  a  property  made  use  of  in  dyeing  with 
this  substance  (p.  507) ;  concentrated  sulphuric  acid  dis- 
solves indigo  with  formation  of  indigodisulplionic  acid, 
C16H8N202(S03H)2,  the  sodium  salt  of  which  is  used  in 
dyeing  under  the  name  *  indigo  carmine.' 

Indigo  has  been  synthetically  produced  by  Baeyer  by 
various  reactions,  two  of  the  more  important  of  which  are 
mentioned  on  pp.  408  and  433. 


CHAPTER     XXX  Y. 

STEREO-ISOMERISM. 

The  constant  use  of  graphic  formulae  in  studying  carbon 
compounds  was  strongly  recommended  in  an  early  chapter 
(part  i.  p.  53),  because,  as  was  then  pointed  out,  such  formulae 
afford  a  fairly  sure  and  complete  summary  of  the  chemical 
properties  of  the  substances  which  they  represent,  whereas 
the  ordinary  molecular  formulae  express  little,  and  are  besides 
more  difficult  to  remember.  The  true  significance  of  graphic 
formulae  was  also  explained;  the  lines  which  are  drawn 
between  any  two  atoms  simply  express  the  conclusion  that, 


STEREO-ISOMERISM.  529 

as  far  as  can  be  ascertained  experimentally,  these  particular 
atoms  are  directly  united,   without   attempting   to  give  the 
slightest  indication  of  the  nature  of   this  union,  or  of   the 
direction  in  which  the  force  of  affinity  is  exerted. 
AVhen,  therefore,  formulae  such  as  the  following 

H  H  H 

I  I  I 

H— C— H  H— C— Cl  H— C— OH 

I  I  I 

H  Cl  H 

are  employed,  it  must  not  be  supposed  that  they  give  any 
idea  whatever  of  the  actual  form  of  the  molecule,  or  intend 
to  indicate  that  all  the  atoms  in  the  molecule  lie  in  one  plane 
(that  is,  the  plane  of  the  paper) ;  such  an  assumption  is  unsup- 
ported by  facts,  and  is,  moreover,  shown  to  be  incorrect  by 
many  considerations,  of  which  the  following  may  be  men- 
tioned. 

(a)  Experience     has     shown     that     methylene    chloride, 
CH2C12,  exists  in  only  one  form,  and  all  attempts  to  obtain 
an  isomeride  have  failed;  yet,  if  a  compound  of  this  com- 
position were  actually  represented  by  the  above  plane  formula, 
it    should   be    capable   of    existing    in    two   isomeric   forms 
— namely, 

H  H 

H— C— Cl  and  Cl— C— Cl 

Cl  H 

because  in  one  case  the  chlorine  atoms  would  be  adjacent, 
in  the  other  they  would  be  separated  by  hydrogen  atoms,  and 
the  relative  positions  of  all  the  atoms  not  being  identical,  the 
substances  themselves  could  not  be  so. 

(b)  Again,     only     two     isomeric    dichlorethanes — namely, 
CH2C1-CH2C1    and    CH3-CHC12,    are    known,    whereas,    if 
ethane  and  its  derivatives  were  actually  composed  of  atoms, 

2  H 


530  STERRO-ISOMERISM. 

all  of  which  lie  in  one   plane,  the  following  five  isomeric 
dichlorethanes  should  be  capable  of  existence  : 

H    H  H    H  H    Cl 

H— C— C— H          H— C— C— Cl          H— C— C— H 

Cl  Cl  Cl  H  Cl  H 

H    Cl  H    Cl 

II  II 

H— C— C— Cl  H— C— C— H 

u         u 

These,  and  a  great  many  other  similar  cases,  show  con- 
clusively that  the  atoms  in  the  molecule  of  a  carbon  com- 
pound cannot  lie  in  one  plane;  were  this  so,  it  would  be 
impossible  to  explain  the  fact  that  a  large  number  of  isomerides 
which,  theoretically,  would  be  capable  of  existence,  have 
never  yet  been  prepared. 

If,  then,  an  attempt  be  made  to  account  satisfactorily  for  the 
known  isomerism  of  carbon  compounds,  it  is  found  that  this 
can  be  done  by  assuming  that  each  of  the  several  atoms  or 
groups  with  which  a  carbon  atom  is  united  is  situated  at  some 
point  on  one  of  four  different  lines,  which  are  symmetrically 
arranged  in  the  space  around  the  carbon  atom.  In  other 
words,  it  may  be  supposed  that  the  carbon  atom  is  situated 
in  the  centre  of  an  imaginary  regular  tetrahedron,  and  that 
its  four  affinities  (those  forces  by  virtue  of  which  it  unites 
with  four  atoms  or  groups)  act  in  the  directions  of  straight 
lines  drawn  from  the  centre  of  the  tetrahedron  to  the  four 
corners,  as  represented  by  the  dark  lines  in  the  following 
figure : 


STEREO-ISOMERISM. 


531 


Xow  this  highly  important  theory,  which  was  advanced  by 
Le  Bel  and  van't  Hoff,  independently,  in  1874,  is  not  based 
solely  on  the  fact  that  it  explains  the  non-existence  of  a  larger 
number  of  isomerides  of  a  given  substance  than  is  actually 
known ;  it  is  also  supported  by  positive  evidence  of  a  very 
weighty  character,  and}  indeed,  may  be  shown  to  accord  well 
with  all  known  facts. 

If,  then,  this  theory  be  applied  in  the  case  of  some  of  the 
simplest  organic  compounds,  it  leads  to  the  following  con- 
clusions : 

(1)  Assuming  that  one  of  the  hydrogen  atoms  in  marsh- 
gas,  CH4,  is  displaced  by  an  atom  X,  there  can  only  be  one, 
substitution    product   of   the   type   CH3X,    because    all    the 
hydrogen  atoms  are  identically  situated. 

(2)  Only  one  di-substitution  product  of  the  type  CH2XY, 
such  as  CH2C12  or  CH2ClBr  (in  which  X  and  Y  are  either 
identical  or  dissimilar),  is  also  possible,  formulas  such  as 


being  absolutely  identical,  although  they  may  appear  to  be 
different  on  paper. 

Points  such  as  these  can  only  be  clearly  understood  by 
actually  handling  models  made  to  represent  arrangements  of 
this  kind ;  *  it  will  then  be  seen  at  once  that,  in  whatever 
manner  the  positions  of  the  different  atoms  H  H  X  Y  are 

*  In  order  to  facilitate  the  study  of  stereochemistry,  sets  of  models 
similar  to  those  recommended  by  Friedlander  have  been  specially  prepared 
at  the  authors'  request  by  Messrs  Baird  and  Tatlock  (14  Cross  Street, 
Hatton  Garden,  London,  E.C.),  from  whom  they  may  be  obtained  at  a  cost 
of  eighteen  pence.  Such  sets  contain  sufficient  models  for  the  study  of  the 
isomerism  of  the  tartaric  acids,  but  larger  sets  adapted  for  the  study  of  the 
sugars  may  also  be  obtained. 


532 


STEREO-ISOMERISM. 


varied,  only  one  arrangement  is  possible,  the  apparent  differ- 
ence which  exists  on  paper  vanishing  at  once  on  rotating  the 
models. 

(3)  In  the  case  of  the  tri-substitution  products  of  methane, 
also,  one  form  only  is  possible,  where  any  two  of  the  sub- 
stituting atoms,   or  groups  of  atoms,   are  the  same,  as,   for 
example,  in  the  compounds 

CHC13        (CH3)2CH.  OH        (C2H5)2CH .  CH2-  OH. 
In  all  these  cases  there  is  perfect  agreement  between  fact 
and  theory,  compounds  of  the  given  types  being  known  in 
one  form  only. 

(4)  If,  however,  three  atoms  in  marsh-gas  be  substituted  by 
three  different  groups,  compounds  of  the  type  C,  H,  X,  Y,  Z*— 
in  which  the  carbon  atom  is  united  with  four  different  atoms 
or  groups — being  obtained,  then  it  is  possible  to  construct  two, 
but  only  two,  different  arrangements,  which  cannot  be  made 
to  coincide  by  rotation,  or  in  any  other  way ;  these  two  forms 
may  be  represented  by  the  following  figures : 


Z  Z  Y 

In  working  with  the  models  this  is  very  clearly  seen,  by  first 
inserting  the  red,  white,  blue,  and  yellow  balls  into  the  two  india- 
rubber  carbon  models,  in  such  a  way  as  to  produce  identical 
arrangements ;  by  then  interchanging  any  two  of  the  balls  in  one 
of  the  models,  a  form  will  be  obtained  which  is  different  from,  and 
which,  therefore,  cannot  be  made  to  coincide  with,  the  other  form 
by  rotating. 

These  two  arrangements  are  related  to  one  another,  in  the 
same  way  as  an  object  to  its  mirror-image — that  is  to  say,  if 
one  be  held  before  a  mirror,  the  position  of  X,  Y,  and  Z  in 
relation  to  H  in  the  mirror-image  will  be  found  to  be 

*  Or  C,  r,  6,  ?0,  y;  compare  foot-note,  p.  536. 


STEREO-ISOMERISM.  533 

identical  with  those  in  the  other  viewed  directly,  an  interesting 
point,  which  again  is  much  more  clearly  seen  by  using  models  ; 
for  the  sake  of  convenience,  one  of  these  arrangements  may  be 
denoted  by  +  ,  the  other  by  -  ,  the  actual  choice  being  im- 
material. 

When,  therefore,  a  carbon  atom  is  united  to  four  different 
atoms  or  groups,  H,  X,  Y,  and  Z,  the  compound  which  is  pro- 
duced may,  theoretically,  exist  in  two  distinct  modifications, 
related  to  one  another  in  the  same  way  as  an  object  to  its 
mirror-image.  Any  carbon  atom  united  in  this  way  is  called 
an  '  asymmetric  carbon  atom,'  on  account  of  its  unsymmetrical 
or  asymmetrical  nature. 

Now  certain  substances,  such  as  active  amyl  alcohol,  sarco- 
lactic  acid,  malic  acid,*  and  mandelic  acid  (p.  440),  which 
have  already  been  described,  have  the  property  of  rotating 
the  plane  of  polarised  light,  and  experience  has  shown  that 
all  substances  which  have  this  property,  when  in  a  liquid 
state,  or  in  solution,  exist  in  (at  least)  two  forms,  one  of 
which  rotates  the  plane  of  polarisation  to  the  right,  the 
other  doing  so  to  precisely  the  same  extent  to  the  left. 

On  considering  the  constitutional  formulae  of  such  optically 
active  organic  substances,  one  remarkable  fact  is  brought  to 
light  —  namely,  that  the  molecule  always  contains  at  least 
one  asymmetric  carbon  atom,  as  is  indicated  in  the  follow- 
ing formula?,  in  which  the  symbol  of  this  particular  carbon 
atom  is  printed  in  heavy  type  : 

CH,H 


3X, 
H\;H- 


C2H;H2-OH  H/ 

Active  Amyl  Alcohol.  Lactic  Acid. 

CH2.COOH  C6H5\       OH 


\/ 


OH          COOH 

Malic  Acid.  Mandelic  Acid. 

*  These  three  compounds  are  described  in  part  i.  pp.  105,  227,  239. 


534  STEREO-ISOMERTSM. 

That  this  property  of  rotating  the  plane  of  polarised  light  is 
due  to  the  presence  in  the  molecule  of  an  asymmetric  carbon 
atom  is  practically  proved  by  the  fact  that  all  optically  active 
compounds  of  known  constitution  contain  a  carbon  atom 
united  in  this  way,  arid  also  by  the  fact  that  if  by  any  means 
the  asymmetric  character  of  the  carbon  atom  be  destroyed,  the 
power  of  rotating  the  plane  of  polarised  light  also  disappears. 
Sarcoladic  acid,  for  example,  is  optically  active,  but  when 
reduced  with  hydriodic  acid,  it  yields  propionic  acid,  which  is 
inactive,  because  it  does  not  contain  a  carbon  atom  united 
with  four  different  atoms  or  groups, 

OH  CH3v/H 

H/  \COOH  H/  \COOH 

Active,  Inactive. 

Malic  acid,   again,  is  optically  active,  but,  on  reduction,  in- 
active succinic  acid  is  formed, 


H\  /CH2.COOH  H        CH2-COOH 

^5? 

OETXJOOH 


^-^ 

^ 

Active.  Inactive. 


A   still    more    instructive    case    is   afforded  by  active  amyl 
alcohol,  and  the  following  derivatives  : 


3x/ 


-OH  C2H/  \CH2T 

Amyl  Alcohol.  Amyl  Iodide. 


3\^  / 

C 
C2H         CH2.CN  C2H/  XCH2.COOH 

Amyl  Cyanide.  Methylethylpropionic  Acid. 

These  substances,  prepared  from  active  amyl  alcohol  by  the 


STEREO-ISOMERISM.  535 

usual  series  of  reactions,  are  themselves  optically  active,  be- 
cause they  still  contain  an  asymmetric  carbon  atom ;  if,  how- 
ever, the  iodide  be  reduced  to  the  hydrocarbon 
CH3\/H 

C2H /  \CH3 

Dimethylethylmethane. 

the  asymmetric  character  of  the  carbon  atom  is  destroyed, 
and  a  substance  is  formed  which  is  optically  inactive. 

This  relation  between  the  presence  of  an  asymmetric  car- 
bon atom  and  the  property  of  rotating  the  plane  of  polarised 
light,  was  first  pointed  out  by  Le  Bel  and  van't  Hoff,  and  is 
now  supported  by  such  a  mass  of  evidence  that  it  may  be 
regarded  as  established. 

Considering  now  some  of  the  simplest  optically  active 
substances — namely,  those  containing  only  one  asymmetric 
carbon  atom,  it  may  be  repeated  that  they  invariably  exist  in 
two  optically  active  forms,  one  of  which  is  dextrorotatory 
(d  or  + ),  the  other  levorotatory  (/or  - )  to  exactly  the  same 
extent.  These  two  forms  are  called  optical,  physical,  or  stereo- 
chemical  i8omerid.es ;  they  have  the  same  chemical  properties 
and  chemical  constitution,  because  their  molecules  differ  only 
as  regards  the  arrangement  in  space.  They  have  also  the 
same  melting-point  and  boiling-point,  and  are  identical  in 
other  physical  properties,  except  that  they  almost  invariably 
differ  to  a  greater  or  less  extent  in  crystalline  form,  inasmuch 
as  the  crystals  of  the  one  are  to  those  of  the  other  as  an 
object  to  its  mirror-image  (p.  540). 

When  any  substance  containing  one  asymmetric  carbon 
atom  is  prepared  synthetically,  the  product  is  found  to  be 
optically  inactive.  When,  for  example,  lactic  acid  is  produced 
from  a-bromopropionic  acid,  or  malic  acid  from  bromosuccinic 
acid  (part  i.  pp.  226  and  240),  the  product  in  each  case  has 
no  action  on  polarised  light. 

This  is  due  to  the  fact  that  the  product  contains  equal 
quantities  of  the  d  and  /  forms,  and  the  action  on  polarised 


536 


STEREO-ISOMERISM. 


light  of  the  one  is  exactly  counterbalanced  by  that  of  the 
other.  This  can  be  proved  by  simply  dissolving  together 
equal  quantities  of  the  d  and  I  forms,  and  then  evaporating 
the  solution,  when  an  inactive  product,  identical  with  that 
produced  synthetically,  is  obtained. 

When,  moreover,  this  inactive  product  is  a  solid,  it  is 
found,  as  a  rule,  to  differ  very  considerably  from  the  active 
forms  in  physical  properties  ;  it  has  a  different  melting-point 
(usually  a  higher  one),  different  solubility,  and  a  different 
crystalline  form,  and  is  spoken  of  as  the  racemic  (inactive  or 
i.r.)  modification  of  the  compound.  Liquid  racemic  modifica- 
tions are  not  known,  and  it  is  doubtful  whether  they  are 
capable  of  existing. 

The  above  statements  refer  simply  to  compounds  containing 
only  one  asymmetric  carbon  atom.  No  matter  how  many 
carbon  atoms  the  molecule  may  contain,  or  what  the  nature 
of  the  other  atoms  may  be,  as  long  as  only  one  of  the  carbon 
atoms  is  combined  with  four  different  atoms  or  groups,  the  com- 
pound exists  only  in  the  above  three  optically  different  forms 
— namely,  d,  I,  and  i.r.  ;  a  substance  of  the  constitution 

H 
CH3.CH2.CH2-CH2— C— COOH, 

OH 

for  example,  would  not  form  a  larger  number  of  optical 
isomerides  than  a  simple  substance  such  as  lactic  acid. 

When,  however,  a  compound  contains  two  asymmetric 
carbon  atoms,  a  larger  number  of  modifications  may  exist  in 
accordance  with  the  above  theory,  as  will  be  seen  at  once  by 
constructing  models  in  the  following  manner  : 

I.  Make  two  identical  asymmetric  carbon  atoms,  C,  r,  b,  to,  yt* 
each  of  which,  for  convenience,  may  be  designated  +;  now 
remove  y  from  both  models,  join  the  two  open  ends  by  means 

*  The  letters  r,  6,  w  and  y  refer  to  the  red,  blue,  white,  and  yellow  balls  in 
the  sets  of  models. 


STEREO-ISOMERISM.  537 

of  the  rod,  and  lay  the  model  on  the  table,  so  that  the  two 
red  balls  point  upwards.     This  is  one  possible  modification,  a 
plane  figure  of  which  may  be  obtained  by  pressing  the  red 
balls  outwards  on  the  table,  when  it  will  appear  like  this  : 
r 

«;—•—&  + 

or 

-10 


MODIFICATION  L 

The  removal  of  one  of  the  balls,  representing  one  of  the 
atoms  or  groups,  and  the  substitution  for  it  of  the  more 
complex  group  (C,  ?',  b,  w),  still  leaves  each  carbon  atom 
asymmetrical ;  in  other  words,  each  is  now  combined  with 
the  four  different  groups  (b),  (w),  (r),  and  (C,  r,  b,  iv\  instead 
of  with  (r),  (b),  (w),  and  (y). 

II.  Repeat  the  above  operations,  starting,  however,  with 
two  identical  asymmetric  carbon  atoms,  C,  r,  b,  y,  w,  which 
are  the  mirror-images  of  those  taken  in  (I.),  and  which  may, 
therefore,  be  called  —  ;  the  plane  representation  of  this  model 
will  be 

r 

i 

*— •— w  — 

_'_       or 


MODIFICATION  II. 

This  form  is  quite  different  from  L,  because  the  one  can- 
not possibly  be  converted  into  the  other  by  rotation ;  if, 
for  example,  II.  be  turned  over,  the  positions  of  b  and  w  will 
correspond  with  those  in  I.,  but  although  the  flat  images 
would  be  the  same,  the  two  are  not  identical,  because  r,  r  will 


538  STEREO-ISOMEfclSM. 

now  point  downwards  in  II.,  whereas  they  pointed  upwards 
in  I.  ;  if,  in  fact,  this  model  (II.)  be  held  before  a  mirror,  it 
will  be  seen  that  it  is  not  identical  with  its  mirror-image,  but 
that  its  mirror-image  is  identical  with  I.  viewed  directly. 

III.  If  now  two  different  asymmetric  carbon  atoms, 
C,  r,  b,  w,  y,  and  C,  r,  b,  y,  w,  or  +  and  — ,  be  joined  in  the 
same  manner  as  before,  another  modification  will  be  obtained 
which  is  quite  different  from  I.  and  II.,  and  which  may  be 
represented  thus : 

r 

I 

w— •— b  + 

I  or 

w — • — b  — 


MODIFICATION  III. 

No  other  forms  different  from  these  three  can  be  con- 
structed. It  is  evident,  then,  that  a  compound  containing 
two  asymmetric  carbon  atoms  may  form  three  distinct 
modifications.  One  of  these  (I.)  will  be  dextrorotatory, 
because  it  contains  two  identical  (  +  )  asymmetric  carbon 
atoms ;  the  other  (II.)  will  be  levorotatory  to  exactly  the 
same  extent,  because  it  contains  two  identical  (-)  asym- 
metric carbon  atoms.  The  third  form,  on  the  other  hand, 
will  be  optically  inactive  j  the  molecule  which  it  represents 
contains  two  different  asymmetric  carbon  atoms,  one  + 
and  the  other  — ,  and  consequently  the  dextrorotatory  action 
of  the  one  is  exactly  counterbalanced  by  the  levorotatory 
action  of  the  other ;  in  other  words,  the  rotatory  power  of 
one  part  of  this  molecule  is  compensated  or  neutralised  by 
that  of  the  other  part ;  such  a  compound  is  said  to  be  inactive 
by  internal  compensation. 

There  is,  however,  a  fourth  modification  which  has  not 
yet  been  considered  in  the  present  case  ;  by  dissolving  equal 
quantities  of  the  two  active  (d  and  I)  forms,  and  then  evap- 


STEREO-ISOMERISM.  539 

orating,  ail  inactive  or  racemic  modification  may  be  obtained, 
just  as  in  the  case  of  the  lactic  acids,  &c.,  and  this  form  is 
said  to  be  inactive  by  external  compensation,  the  action  of  two 
separate  molecules  counterbalancing  one  another. 

In  order  to  decide  which  two  of  the  above  three  forms  represent 
the  active  (d  and  I)  modifications  of  the  substance,  it  is  only 
necessary  to  determine  which  two  models  behave  to  each  other 
as  object  to  mirror-image.  This  will  be  found  to  be  the  case  with 
the  forms  I.  and  II.,  which  are  therefore  the  active  forms;  on  the 
other  hand,  the  form  III.  coincides  with  its  own  mirror-image,  and 
is,  therefore,  inactive. 

The  same  conclusions  are  arrived  at  by  disconnecting  and  then 
comparing  the  asymmetric  carbon  atoms,  when  it  is  easy  to  see  that 
one  of  the  models  is  composed  of  two  different  arrangements  ;  this, 
therefore,  is  the  form  which  is  inactive  by  internal  compensation. 

Stereo-isomensm  of  the  Tartaric  Acids. 

One  of  the  best  examples  of  the  stereo-isomerisni  of  sub- 
stances containing  two  asymmetric  carbon  atoms  is  that  of  the 
tartaric  acids,  COOH-CH(OH)-CH(OH).COOH.  As  will  be 
seen  from  the  constitutional  formula,  there  are  two  carbon  atoms, 
each  of  which  is  united  with  four  different  atoms  or  groups — 
namely,  {COOH},  {H},  {OH},  and  {CH(OH)-COOH}, 
and  consequently,  theoretically,  there  should  be  four  physically 
isomeric  forms  of  this  acid. 

As  a  matter  of  fact,  four  modifications  are  known — namely, 
dextrotartaric,  levotartaric,  mesotartaric  and  racemic  acid, 
(part  i.  p.  245). 

Dextrotartaric  acid  and  levotartaric  acid  are  the  two 
optically  active  modifications,  and  may  be  respectively  repre- 
sented by  the  formulae, 

COOH  COOH 

I  I 

H— C— OH  +       OH— C— H   - 
I  and 

[— C- 


OH— C— H   +        H— C-OH 

I  I 

COOH  COOH 


540 


STEREO-ISOMERISM. 


The  one  rotates  the  plane  of  polarisation  to  the  right  to 
exactly  the  same  extent  as  the  other  to  the  left ;  but  in  all 
other  respects  they  are  identical,  except  for  slight  differences 
in  crystalline  form.  They  possess  the  same  melting-point, 
and  the  same  solubility  in  various  solvents ;  their  metallic 
salts  have  the  same  composition,  and  crystallise  with  the  same 
number  of  molecules  of  water.  Their  ethereal  salts  melt  and 
boil  at  the  same  temperature ;  all  their  salts,  like  the  acids 
themselves,  are  optically  active  to  the  same  extent,  but  in 
opposite  directions. 

In  addition  to  this  difference  in  their  action  on  polarised 
light,  these  two  active  tartaric  acids  and  the  corresponding 
salts  show  a  slight  difference  in  crystalline  form,  which  is 
exhibited  very  clearly  in  the  case  of  the  well-defined  crystals 
of  their  sodium  ammonium  salts,  C4H406Na(NH4)  +  4H20. 


Fig.  21. 


If  these  crystals  be  examined,  it  will  be  found  that  certain 
faces  (those  which  are  darkened  in  the  figures)  which  are  on 
the  right-hand  side  of  the  crystals  of  the  dextrorotatory  acid, 
are  on  the  left-hand  side  of  those  of  the  levorotatory  acid.  The 
two  kinds  of  crystals  are,  in  fact,  related  as  an  object  to  its 
mirror-image,  as  will  be  seen  by  holding  i.  before  a  mirror, 
when  the  darkened  faces  will  appear  as  in  u.  viewed  directly, 
and  vice  versd.  A  similar  difference  in  the  crystalline  form  is 
observed  in  the  case  of  other  optically  active  substances,  and 
such  crystals  are  said  to  be  enantiomorphous. 

Mesotartaric  acid,  C4H606,  is  the  simple  optically  inactive 


STEREO-1SOMERISM.  541 

form  of  tartaric  acid ;  that  is  to  say,  it  is  inactive  by  internal 
compensation  (see  above),  and  may  be  represented  by  the 
formula, 

COOH 

I 
H— C— OH     + 

H— C— OH     - 


It  differs  from  the  two  optically  active  forms  in  many  re- 
spects, as,  for  example,  in  melting-point,  solubility,  and 
crystalline  form.  It  might,  in  fact,  be  regarded  as  quite  a 
different  substance  from  an  examination  of  its  physical  pro- 
perties, and  of  those  of  its  salts,  although,  in  chemical  pro- 
perties, it  is  identical  with  the  active  forms.  On  the  other 
hand,  mesotartaric  acid  resembles  racemic  acid  very  closely 
in  physical  properties,  but,  unlike  the  latter,  it  cannot  be 
resolved  into  two  optically  active  modifications,  because  it  is  a 
simple  substance. 

Racemic  acid,  C4H606,C4H606,  is  the  double  inactive 
form  of  tartaric  acid,  and  is  simply  composed  of  equal  quanti- 
ties of  dextro-  and  levo-tartaric  acids ;  that  is  to  say,  it  is 
inactive  by  external  compensation  (see  above),  and  may  be 

•   r\  TT  Q     4-4- 

represented  by    the  formula   \  r^^fr\  •       ^    a^so    ^e- 

I  L/4±16U6  ~ 

haves  as  if  it  were  a  distinct  substance,  as  far  as  physical 
properties  are  concerned,  which  is  all  the  more  remarkable 
when  it  is  borne  in  mind  that  racemic  acid  is  obtained  on 
evaporating  a  solution  of  equal  quantities  of  the  two  active 
modifications,  and  that  it  can  be  again  separated  into  these 
two  forms  by  the  methods  given  below. 

It  will  be  seen  from  the  above  examples  that  the  existence 
of  physical  isomerides,  and  the  number  of  such  modifications, 
is  in  complete  accordance  with  the  theory  of  Le  Bel  and  van't 
Hoff,  and  a  great  many  other  cases  might  be  mentioned  in 
which  the  agreement  is  quite  as  perfect. 


542 


STEREOISOMERISM. 


As  the  number  of  asymmetric  carbon  atoms  increases,  the 
number  of  isomerides  naturally  becomes  larger,  so  that  a 
substance  such  as  saccharic  acid  (part  i.  pp.  264,  270), 

COOH.CH(OH)-CH(OH).CH(OH).CH(OH).COOH, 

which  contains  four  asymmetric  carbon  atoms,  is  capable  of 
existing  in  ten  optically  isomeric  forms  (which  may  be  con- 
structed with  the  aid  of  models). 

As  in  the  case  of  chemical  isomerism,  however,  all  the 
theoretically  possible  isomerides  of  a  given  substance  have  not 
always  been  actually  obtained  owing  to  experimental  diffi- 
culties ;  dimethylsuccinic  acid, 

COOH.CH(CH3).CH(CH3).COOH, 

for  example,  like  tartaric  acid,  should  exist  in  four  forms,  but 
only  two  are  known,  both  of  which  are  optically  inactive,  the 
two  active  forms  not  having  yet  been  isolated. 

An  examination  of  the  models  of  substances  containing  two 
asymmetric  carbon  atoms — that  is,  of  substances  derived  from  the 
symbol, 


might  lead  to  the  supposition  tliat  they  should  exist  in  more  than 
four  modifications. 

In  the  first  place,  the  model  could  be  so  arranged  that  the 
directions  of  the  affinities  of  the  two  carbon  atoms  would  be  as 
shown  in  the  figure.  If,  then,  one  of  the  carbon  atoms  were  slowly 


STEREO-ISOMERISM.  543 

rotated  about  an  axis,  an  infinite  number  of  forms  would  be  pro- 
duced, all  of  which  would  be  different,  because  they  would  represent 
different  relative  positions  in  space  of  the  atoms  constituting  the 
molecule.  It  would  be  just  the  same  even  if  the  substance  did 
not  contain  an  asymmetrical  carbon  atom  ;  ethane,  CH3-CH3,  or 
ethylene  chloride,  CH2C1-CHSC1,  for  example,  could  in  this  way 
be  represented  as  existing  in  an  infinite  number  of  modifications. 

This  objection,  however,  at  once  disappears  on  considering  the 
matter  a  little  more  carefully. 

In  a  compound  represented  by  the  above  symbol  (by  attaching 
atoms  or  groups  to  the  corners  of  the  imaginary  tetrahedra),  the 
atoms  or  groups  united  with  one  of  the  carbon  atoms  must  exert 
a  certain  attraction  or  repulsion  on  those  united  with  the  other, 
those  which  have  the  greatest  affinity  for  each  other  striving  to 
approach  as  nearly  as  possible,  until  a  certain  position  of  equilibrium, 
which  is  the  resultant  of  all  the  mutual  attractions,  is  reached. 

This  position  may  be  disturbed  by  the  application  of  heat  or  of 
some  other  force,  but  on  removing  the  disturbing  element,  the 
original  form  will  be  restored,  so  that,  under  given  conditions,  the 
compound  only  exists  in  one  form,  unless,  of  course,  it  contains 
asymmetric  carbon  atoms. 

Resolution  of  Racemic  Modifications. 

The  racemic  modification  of  tartaric  acid  and  the  corre- 
sponding forms  of  other  optically  active  substances — namely, 
of  those  which  are  inactive  because  they  are  composed  of  equal 
quantities  of  the  two  opposed  active  forms — may  sometimes  be 
resolved  into  their  components  by  one  or  other  of  the  follow- 
ing methods  : 

(1)  By  crystallisation  of  the  salt  formed  by  the  combina- 
tion of  a  racemic  acid  or  base  with  an  optically  inactive  sub- 
stance. This  method  was  first  employed  by  Pasteur  in  the 
case  of  racemic  (tartaric)  acid,  and  depends  on  the  fact  that  if 
a  solution  of  sodium  ammonium  racemate  be  allowed  to 
crystallise  at  a  particular  temperature  (below  28°),  enantio- 
morphous  crystals  (right-  and  left-handed,  as  shown  in  the 
fig.,  p.  540)  are  deposited.  If  now  these  crystals  are  sorted 
mechanically,  the  right-handed  ones  being  placed  in  one 
vessel,  the  left-handed  ones  in  another,  a  separation  of  the 


544  STEREO-ISOMERISM. 

racemic  acid  into  its  constituents  is  accomplished,  one  kind 
of  crystals  being  those  of  the  salt  of  the  dextro-acid,  the  other 
those  of  the  salt  of  the  levo-acid.  If,  however,  crystallisation 
take  place  at  temperatures  above  28°,  only  one  kind  of  crystal 
is  deposited — namely,  crystals  of  sodium  ammonium  racemate, 
which  do  not  exist  in  enantiomorphous  forms,  and  which, 
indeed,  belong  to  quite  a  different  crystalline  system.  This 
method  of  separation  is  not  applicable  in  all  cases,  because,  as 
a  rule,  the  crystals  of  the  salts  of  the  two  active  components 
are  not  sufficiently  well  defined  to  allow  of  their  mechanical 
separation,  even  if  they  are  deposited  separately. 

(2)  A  second   method,  also  discovered  by  Pasteur,  consists 
in  fractionally  crystallising  the  salt  formed   from  a  racemic 
acid  or  base  with  an  optically  active  substance.     This  method 
depends  on  the  fact,  that  the  two  constituents  of  the  racemic 
modification,   form,  with  one  and  the  same  optically  active 
substance,  salts  which  differ  in  solubility,  and  which,  there- 
fore,  can   be   separated  by    fractional  crystallisation  in    the 
ordinary  way.     If,   for   example,  racemic   acid  be  combined 
with    the    optically   active    base    cinchonine     (p.    493)     or 
strychnine  (p.  494),   the  product  may  be  resolved  into  the 
salts  of  the  dextro-  and  levo-acids ;  in  a  similar  manner  the 
inactive  modification  of  coniine  (p.  489)  may  be  resolved  into 
its  constituents  by  fractional  crystallisation  of  the  salt  which 
it  forms  with  dextrorotatory  tartaric  acid. 

(3)  Another  method    of    separation,    quite    different    in 
principle  from  the    foregoing,    depends    on  the  fact  that  if 
certain  organisms,  such  as'  penicillium  glaucum,  be  placed  in  a 
solution  of  a'racemic  modification,  they  feed  on  and,  therefore, 
destroy    one — usually   the   dextro — modification,    the    result 
being  that,  after  a  time,  the  solution  contains  only  the  levo- 
isomeride. 


INDEX. 


[Where  more  than  one  reference  is  given,  and  one  of  them  is  in  heavy  type,  the  latter 
refers  to  the  systematic  description  of  the  substance.] 


PAGE 

Acetanilide 360,  362 

Acetophenone 411 

Acetophenonehydrazone 412 

Acetophenoneoxime 412 

Acetotoluidide 360 

Acetylbenzene 411 

Acetylcodeine 497 

Acid  dyes,  506  ;  Acid  green 511 

AcroleTn 482 

Acrylaniline 482 

Active  amyl  alcohol 533 

Alizarin,  464,  465 ;  constitution  of, 
467;  diacetate,  467  ;  dyeing  with. ..504 

Alkali  blue 518 

Alkaloids,  484 ;  extraction  of 488 

Alkaloids,  contained  in  opium,  495  ; 
derived  from  pyridine,  488 ;  derived 
from  quinoline,  492  ;  related  to  uric 

acid 498 

Alkylanilines 364 

Amalinic  acid. 498 

Amidoazobenzene,  375,  522,  524 ; 
hydrochloride,  375  ;  sulphonicacid  523 

Amidoazo-compounds 374 

Amidoa2Otoluene  hydrochloride 522 

Amidobenzaldehydes 408,  410 

Amidobenzene 361 

Amidobenzenesulphonic  acid,  tn,  o. .  .384 

Amidobenzenesulphonic  acid,  / 383 

Amidobenzoic  acid,  m,  o,  p 422 

Amido-compounds 325,  355 

Amidoethylsulphonic  acid 501 

Amidonaphthalene 444,  451 

«-Amido-,3-naphthol 455 

i  :4-Amidonaphthol 455 

Amidophenol,  / 415 

Amidotoluene 364 

Amygdalin 405 

Amyl  alcohol,  534 ;  cyanide,  534 ; 
iodide 534 

ir. 


2i 


Auethole 410,  439 

Aniline,  361  ;  homologues  of,  364 ; 
hydrochloride,  362 ;  platinochlo- 
ride,  362;  stannichloride,  356,  361  ; 
substitution  products  of,  363 ;  sul- 
phate, 362;  sulphonicacid,/ 383 

Aniline  blue 517 

Aniline  yellow 524 

Animal  charcoal,  use  of. 393 

Anisalcohol 404 

Anisaldehyde 404,  410 

Anisic  acid 404,  41  r,  439 

Anisole 392 

Anisyl  alcohol 410 

Anthracene.. 298.  328,  457 

Anthracene,  constitution  of. 458 

Anthracene  derivatives,  isomerism  of.  461 

Anthracene  dichloride 462 

Anthracene  disulphonic  acids 464 

Anthracene  oil 296,  298 

Anthracene  picrate 458 

Anthranilic  acid 422,  437 

Anthranol 464 

Amhrapurpurin 468 

Anthraquinone,  458,  462  ;  test  for 465 

Anthraquinone-/3-monosulphonic 

acid 464,  466 

Anthraquinonedisulphonic  acid 468 

Anthraquinonesulphonic  acid,  sodium 

salt  of. 464 

Antifebrin 362,  500 

Antipyrine 499 

Arbutin 399 

Aromatic,  alcohols,  385,  402 ;  alde- 
hydes, 405  ;  amines,  355,  368  ;  com- 
pounds, general  properties  of,  322  ; 

halogen  derivatives 341 

Aseptol 396 

Asymmetric  carbon  atom 533 

Atropine,  490;  sulphate  491 ;  test  for.49i 


INDEX. 


PAGE 

Aurin 513,  518 

Azobenzene 378,  522 

Azobenzenesulphonic  acid 522 

Azo-compounds 377 

Azo-dyes,  506,  522;  preparation  of. ..523 

Basic  dyes 506 

Baumann  and  Schotten's  method . . .  .420 

Benzal  chloride 341,  342,  349,  407 

Benzaldehyde,  405  ;  bisulphite  comp.4o6 

Benzaldehyde  green 510 

Benzaldoxime 407 

Benzamide 421 

Benzene,  297,  298;  constitution  of. ..303 
Benzene  derivatives,  constitution  0^.317 
Benzene  derivatives,  isomerism  of.... 310 

Benzene  hexabromide 303 

Benzene  hexachloride 303,  326 

Benzene  hexahydride 326 

Benzene  homologues,  328  ;  properties 

of,  331  ;  oxidation  of. 333 

Benzene,  synthesis  of 301,  324 

Benzene-;«-dicarboxylic  acid 426 

Benzene-0-dicarboxylic  acid 425 

Benzene-/-dicarboxylic  acid 427 

Benzenedisulphonic  acid,  m,  o,  p. . .  .383 

Benzenesulphonamide 383 

Benzen'esulphonic  acid 382 

Benzenesulphonic  chloride 383 

Benzidine 379,  526 

Benzoic  acid,  418  ;  salts  of,  419  ;  sub- 
stitution products  of 422 

Benzoic  anhydride 420 

Benzonitrile 421 

Benzophenone 340,  412 

Benzopurpurin 526 

Benzoquinone 413 

Benzotrichloride 342,  349 

Benzoylaniline 420 

Benzoylbenzene 412 

Benzoylbenzoic  acid,  o 463 

Benzoyl  chloride,  Benzoyl-group  ....  420 
Benzyl,   acetate,   349,    403  ;    alcohol, 
403  ;    bromide,  403 ;  chloride,  340, 
342,  348,   460  ;    cyanide,  422,  429 ; 

ethyl  ether,  418  ;  radicle 333 

Benzylamine 368 

Benzylidene  radicle 407 

Benzylideneacetone 407 

Benzylidenehydrazone 407 

Benzylidenehydroxycyanide 407 

Benzylmalonic  acid 429 

ii 


PAGE 

BetaTne,  500  ;  chloride 501 

Bismarck  brown 524 

Bone-oil,  Bone-tar 472 

Bordeaux 525 

Brilliant  green 511 

Bromacetylene 324 

Bromanthraquinone 463 

Bromobenzene 303,  347 

Bromobenzoic  acid,  m,  o,  p 422 

Bromobenzoylbenzoic  acid 463 

Bromobenzyl  bromide,  o 460,  469 

a-Bromonaphthalene 450 

/3-Bromonaphthalene 450 

Bromonitrobenzene,  m,  o,  p 354 

Bromophthalic  acid  ;  anhydride 463 

Bromotoluene,  o 461 

Brucine,  494,  495 ;  test  for 495 

Brucine,  ethiodide  ;  hydrochloride  ..495 

Butyrolactone 519 

Butyrophenone 412 

Caffeine,  497  ;  hydrochloride 498 

Calico-printing 505 

Carbazole 457 

Carbolic,  acid,  297,  298,  391  ;  oil. ...296 

Carboxylic   acids 416 

Carvacrol 339,  397 

Catechol 398,  467 

Catecholcarboxylic  acid 439 

Catechu 398 

Chloracetanilide 363 

Chloranil 416 

Chloraniline,  tn,  o,  p 363 

Chlorobenzene 303,  347 

Chlorobenzoic  acid 348 

Chlorobenzyl  chloride,  / 343 

Chloronaphthalene,  «-,  449 ;  /3- 450 

Chloronitrobenzene, m,  o,  p,  354  ;  ^..363 

Chjorotoluene,  m,o,p 348 

Choline,  500  ;  chloride 500 

Chrysoidine 524 

Cinchomeronic  acid 483 

Cinchona-bark,  alkaloids  of. 492 

Cinchonine 492,  493 

Cinchoninic  acid 493 

Cinnamic,  acid,  430  ;  aldehyde 405 

Closed-chain  compounds 323 

Coal-tar,  distillation  of. 295,  299 

Coca,  alkaloids  of 491 

Cocaine,  491  ;  hydrochloride 491 

Codeine 495.  497 

Coke...  295 


INDEX. 


PAGE 

Collidines 478 

Colour-base 508 

Congo  group  of  dyes. 5-4,  526 

Congo-red 455,  526 

Coniine,  488  ;  hydrochloride 469 

Creosote  oil 296,  298 

Cresol,  298  ;  Cresol,  m,  o,  p 396 

Cumene 338 

Cumic  acid 338 

Cymene 339 

Dahlia 515 

Daturine 490 

Dextrorotatory  compounds 535 

Dextrotartaric  acid 539 

Diallyl,  Diallyl  tetrabromide 303 

Diamidoazobenzene ;  hydrochloride.. 524 

Diamidobenzene 364 

Diamidobenzene,  m 354 

Diamido-compounds 360 

Diamidodiphenyl,  p 378 

i:4-Diamidonaphthalene 455 

Diamidophthalophenone 520 

Diazoamidobenzene 374 

Diazoamido-compounds 374 

Diazobenzene,  chloride,  371  ;  cyanide, 

372  ;  nitrate,  371  ;  sulphate 371 

Diazobenzenesulphonic  acid  . . .  .384,  523 

Diazo-compounds 325,  344,  370 

Diazo-compounds,  constitution  of 373 

Diazocumene  chloride 525 

Diazotoluene  chloride 372 

Diazoxylene  chloride 525 

Dibasic  acids 423 

Dibenzylamine 369 

a/3-Dibromanthraquinone 465 

Dibromethy  Ibenzene 432 

Dibromopyridine 473 

Dichloranthracene 462 

Dichlorobenzene 303 

Dichloronaphthalene 450 

Diethylaniline 365 

Digallic  acid 440 

Dihydric  phenols 387,  388,  398 

Dihydrobenzene 3<>9>  326 

Dihydroxyanthraquinones 468 

«/3-Dihydroxyanthraquinone 465 

Dihydroxyazobenzene' 522 

Dihydroxybenzene,  m,  o,  398  ;  p 399 

Dihydroxybenzoic  acids 439 

i:2-Dihydroxynaphthalene 456 

1 14-Dihydroxy  naphthalene 456 


PAGE 

Dihydroxyphenanthrene 470 

Dihydroxyphthalophenone 519 

Dimethylainidoazobenzene 376 

Dimethylamidoazobenzene        hydro- 
chloride 522 

Dimethylamidoazobenzenesulphonic 

acid 524 

Dimethylaniline 358,  366 

Dimethylbenzidine 379 

Dimethylcatechol 398 

Dimethylethylmethane 535 

Dimethyl-/-phenylenediamine.  .367,  527 

Dirnethylpyridines 478 

a-Dmaphthol,  /3-Dinaphthol 454 

Dinitro-«-disulphon:c  acid,  potassium 

salt  of 454 

Dinitro-a-naphthol 454,  527 

Dinitrobenzene,  m,  353;  o,/....353,  354 

Dinitrobenzenes,  constitution  of 318 

Dinitrophthajophenone 520 

Diphenic  acid 469,  470,  471 

Diphenic  anhydride 471 

Diphenyl,  327,  340,  469  ;  ketone,  340,  412 

Diphenylamine 359,  367 

Diphenyldicarboxylic  acid 469 

Diphenylethylene 469 

Diphenylmethane 340,  413 

Diphenyl-w-tolylmethane 511,  515 

Dippel's  oil 472 

Dipropargyl.. 303 

Ditolyl,  o 469 

Dyes  and  their  application 502 

Ecgonine 491 

Enantiomorphous  crystals 540 

Eosin,  521;  potassium  salt  of 521 

Erythrosin 521 

Ethyl,  benzenesulphonate,  381 ;  ben- 

zoate,    419 ;    benzylmalonate,  429  ; 

mandelate,    441  ;    phthalate,   426  ; 

salicy  late 438 

Ethylaniline 3^5 

Ethylbenzene 335,  337 

Ethylbenzylaniline 511 

Fatty  compounds 322 

Fittig's  reaction 330 

Fluorescei'n,  425,  520;  reaction,  399; 

sodium  salt  of. 521 

Formanilide 363 

Friedel  and  Craft's  reaction 329,  411 

Fuchsine 513 

iii 


INDEX. 


PAGE 

Gallic  acid 439 

Gas  liquor 295 

Glucosides 488 

Guaiacol 398 

Gum  benzoin 418 

Heavy  oil 296 

Helianthin 524 

Hemimellitene 338 

Hemlock,  alkaloids  of 488 

Hexahydropyridine 473,  476 

Hexahydrotetrahydroxybenzoic  acid. 492 

Hexamethylene 326 

Hexamethylrosaniline  chloride 515 

Hippuric  acid 418 

Hofmann's  violet 515 

Hydranthracene 460,  461 

Hydrazines 373 

Hydrazobenzene 378 

Hydrobenzamide 408 

Hydrocarbons,  aromatic,  oxidation  of  417 

Hydrocinnamic  acid 430 

Hydroquinone 399,  414 

Hydroxyaldehydes,  aromatic 408 

Hydroxyantbraquinone 463,  466 

Hydroxyazobenzene 522 

Hydroxyazobenzenesulphonic  acid..  .523 
Hydroxybenzaldehyde,  ;;/,  p>  410 ;  o  409 

Hydroxybenzene 391 

Hydroxybenzoic  acid,  <?,  437;  ;;*,/..  438 

Hydroxybenzyl  alcohol,  m,  o,  p 404 

y-Hydroxybutyric  acid 519 

Hydroxycarboxylic  acids 433 

Hydroxyethylsulphonic  acid 502 

Hydroxyethyltrimethylammonium 

hydroxide 500 

Hydroxyhydroquinone 401 

Hydroxymethyltetrahydroquinoline..499 

Hydroxytoluene,  in,  o,  p, 396,  403 

Hyoscine,  Hyoscyamine 490 

Hypnone 412 

Indican 528 

Indigo,    527 ;  carmine,    528  ;    dyeing 

with,  507  ;  synthesis  of 408,  433 

Indigo  white 507,  528 

Indigodisulphonic  acid 528 

Indigotin 528 

Iodine  green 516 

lodobenzene 348 

lodonitrobenzene,  m,  o,  p 354 

Isethionic  acid 502 

iv 


PAGE 

Isonicotinic  acid 479 

Isophthalic  acid 426 

Isopropylbenzene 338 

Isopropylbenzoic  acid 338 

Isoquinoline,  482,  483;  acid  sulphate-483 

Kairine,  499;  hydrochloride 499 

Ketones,  aromatic 411 

Korner's  method  of  determining  con- 
stitution   320 

Lactic  acid 533 

Lactones 519 

Lakes 467,  506 

Laubenheimer's  reaction 470 

Laudanum 496 

Le  Bel  and  van't  Hoff's  theory 530 

Lecithin 500 

Leucaniline 511 

Leuco-base,  508;  Leuco-compounds  .507 

Leuco-malachite  green 509 

Leuco-pararosaniline 511,  512 

Leuco-rosaniline 511,  514 

Levorotatory  compounds 535 

Levotartaric  acid 539 

Liebermann's  reaction 390 

Light  oil 296 

Lutidines  . , 478 

Magenta 513 

Malachite  green,  509;  chloride  of, 
509 ;  hydrochloride  of  base  of,  508, 
509,  510 ;  oxalate  of,  510 ;  zinc 

double  salt  of , 510 

Malic  acid 533,  534 

Mandelic  acid 440,  533 

Martins'  yellow 454,  527 

Meconic  acid 495 

Mesitylene,  337  ;  constitution  of  319,  324 

Mesitylenic  acid 338 

Mesotartaric  acid 539,  540 

Meta-compounds 313 

Metanilic  acid 384 

Methoxyaniline,  / 499 

Methoxybenzaldehyde,  / 410 

Methoxybenzoic  acid,  p 439 

Methoxybenzoic  acids 397 

Methoxybenzyl  alcohol,  p. .404 

Methoxycinchonine , 492 

Methoxy-group 485 

Methoxyquinoline 499 

Methoxyquinoline-y-carboxylic  acid  .492 


INDEX. 


PAGE 

Methoxytetrahydroquinoltne 499 

Methyl  isophthalate,  427;  methyl- 

salicylate,   436,   438 ;    orange,  525  ; 

potassiosalicylate,  436  ;    salicylate, 

436,  438  ;  terephthalate,  427 ;  violet. 516 

Methylacetanilide 365 

Methylaniline 357,  366 

Methylbenzene 334 

a-Methylcinnamic  acid 431 

Methylcresols 397 

Methylene  blue 527 

Methylethylpropionic  acid 534 

Methy lisopropylbenzene,  / 339 

Methylrmrphine 497 

«-Methylnaphthalene 445 

/3-Methylnaphthalene 449 

Methylpiperidine 477 

Methylpyridines 478 

Methylquinoline 482 

Methylsalicylic  acid 436,  438 

Methyltheobromine 497 

Methyltripheny  Imethane 511 

Middle  oil 296 

Mirbane,  essence  of 353 

Monobromopyridine 473 

Monochloranthracene. 462 

Monohydric  phenols 391 

Monohydroxynaphthalenes,  the 452 

Mordants 504 

Morphine,  496  ;  hydrochloride,  496  ; 

methiodide,  497  ;  tests  for 496 

Naphtha,  crude,  296;  solvent 297 

Naphthalene 297,  298,  328,  442 

Naphthalene,  amido-derivatives  of. .  .451 

Naphthalene,  constitution  of 443 

Naphthalene,  derivatives  of. 449 

Naphthalene   derivatives,    isomerism 

of 447 

Naphthalene,  homologues  of. 449 

Naphthalene,  nitro-derivatives  of. ...451 

Naphthalene  picrate 443 

Naphthalene,  sulphonic  acids  of 454 

Naphthalene  tetrachloride 425,  450 

Naphthalene  yellow 454 

Naphthalenedisulphonic  acids 455 

Naphthalenesulphonic  acids 449,  454 

Naphthalene-«-sulphonic  acid  ..453,  455 
Naphthalene-/3-sulphonic  acid  .  .454,  455 

Naphthalenetrisulphonic  acids 455 

Naphthalic  acid 471 

a-Naphthaquinone -452>  455 


PAGE 

/3-Naphthaquinone 456 

Naphthionic  acid 455,  525,  526 

«-Naphthol,  447,  453  ;  /3-Naphthol. .  .454 

Naphthol  yellow 454,  527 

Naphthol  yellow,  potassium  salt 527 

a-Naphtholdisulphonic  acid 527 

/3-Naphtholdisulphonic  acid 525 

«-Naphtholmonosulphonic  acid 527 

Naphtholmonosulphonic  acids 455 

Naphthols 452 

a-Naphtholtrisulphonic  acid 454,  527 

«-Naphthylamine 452,  453 

/3-NaphthyIamine 452 

Naphthylaminemonosulphonic  acids. 455 

Naphthylamines 449 

i  :4-Naphthylaminesulphonic  acid 455 

Narcotine 495 

Neurine,  501  ;  chloride 501 

Nicotine,    489  ;    dimethiodide,    489 ; 

hydrochloride 489 

Nicotinic  acid 472,  479,  490 

Nightshade,  alkaloids  of 490 

Nitracetanilide,  o,  p 363 

Nitraniline,  t/t,  354  ;  m,  a,  p 363 

a^-Nitroalizarin,  /31-Nitroalizarin 467 

Nitrobenzaldehyde,  m,  o,  p 408 

Nitrobenzene,  352  ;    oxidising  action 

of 480,  514 

Nitrobenzoic  acid,  m,  o,  p 422 

Nitrocinnamic  acid,  o,  p 432 

N  itro-compounds 325,  350 

Nitronaphthalene 444 

«-Nitronaphthalene 451 

/3-Nitronaphthalene 451 

jS-Nitro-oe-naphthylamine 451 

Nitrophenol,  m,  o,  p 392 

Nitrophenyldibromopropionic   acid, 

o,  p 432 

Nitrophenylpropiolic  acid,  o 432 

Nitrophthalic  acid 444 

Nitrosodimethylaniline 366,  367,  527 

Nitrosomethylaniline 366 

Nitrosophenol,  p 367 

Nitrosopiperidine 477 

Nitrotoluene,  in,  o,  p 355 

Nux  vomica,  alkaloids  of 494 

Oil    of    aniseed,    410,    439 ;     bitter 

almonds,  405  ;  wintergreen 437 

Open-chain  compounds 323 

Opium,  496  ;  alkaloids  of 495 

Optical  isomerides 535 

V 


INDEX. 


PAGE 

Optically  active  substances 533 

Organic  compounds,  classification  of.322 

Ortho-compounds 313 

Orthodiketones 470 

Orthoquinones 456 

Osazones 377 

Oxanilide 363 

Oxanthrol 464 

Papaverine 495 

Para-compounds 313 

Paraleucaniline 511,  512 

Paraquinones 456 

Pararosaniline,  511,  512  ;  base  of,  511 ; 
chloride,  511,  513  ;  constitution  of.. 513 

Pentamethylene  diamine 478 

Pentamethylpararosaniline  chloride.  .516 

Pepper,  alkaloids  of 490 

Peri-position 448,  471 

Perkin's  reaction 431 

Peru  balsam 418 

Phenanthraquinone 469,  470 

Phenanthraquinone,  bisulphite  com- 
pound of. 470 

Phenanthraquinone  dioxime 470 

Phenanthrene 298,  457,  468 

Phenanthrene,  constitution  of 470 

Phenetole 392 

Phenol,  297,  391  ;  Phenols 385 

Phenolphthalei'n 519 

Phenolsulphonic  acid,  o,  m, p,  395 ;  /.-384 
Phenyl  benzoate,  420 ;  bromide,  347 ; 
chloride,  347  ;  cyanide,  421  ;   ethyl 
ether,  392 ;  group,  327  ;  iodide,  348  ; 
methyl  ether,  392  ;  radicle. . .  .333,  390 

Phenylacetaldehyde 405 

Phenylacetic  acid 428,  429 

Phenylacetonitrile 422 

Phenylacetylene 432 

Phenylacrylic  acid 428,  430 

Phenylarnine 361 

Phenyl-/3-bromopropionic  acid 431 

Phenylbutylene,  446  ;  dibromide 446 

Phenylbutyric  acid 428 

Phenylcarbinol 403 

Phenylcarbylamine 360,  362 

Phenylchloroform 349 

Phenyl-«/3-dibromopropionic  acid. ...  431 

Phenylene  radicle 333,  390 

Phenylenediamine,  ;«,  354,  524;  /. ..414 

Phenylenediamine,  •m,  o,p 360,  364 

Phenylethane 337 

vi 


PAGB 

Phenylethyl  alcohol 405 

Phenylethylene 432 

Phenylformic  acid 428 

Phenylglycollic  acid 440 

Phenylhydrazine  ;  hydrochloride 376 

Phenylhydrazones 377 

_Phenylhydroxylamine 356 

Phenylisocrotonic  acid 431,  447 

Phenylmethane 334 

Phenylmethyl  carbinol,  412;  ketone..4ii 

Phenylmethylacrylic  acid 431 

Phenylmethylpyrazolone 499 

Phenylpropiolic  acid 428,  432 

Phenylpropionic  acid 428,  430 

Phenyltrimethylammonium  iodide. .  .360 

Phloroglucinol 400,  401 

Phloroglucinol  triacetate 401 

Phloroglucinol  trioxime 401 

Phosphomolybdic  acid 488 

Phosphotungstic  acid 488 

Phthalei'ns,  the 518 

Phthalic  acid 425,  444 

Phthalic  acids,  m,  o,  p 423,  424 

Phthalic  acids,  constitution  of 318 

Phthalic  anhydride 426,  467 

Phthalimide 426 

Phthalophenone 518 

Phthalyl  chloride 426 

Physical  isomerides 535 

Picolines    478 

Picolinic  acid 479 

Picric  acid '. 394,  488,  502 

Piperic  acid 477,  490 

Piperidine,  473,  476;  constitution  of  .477 

Piperine 476,  490 

Pitch 296,  298 

Ponceau  3R 525 

Ponceaux 525 

Potassium  cresate,  390;  diphenyl- 
amine,  368 ;  phenate,  392  ;  phthali- 

mide,  426  ;  picrate 394 

Primula 515 

Propiophenone 412 

«-Propylpiperidine,  d 489 

Protocatechuic  acid 439 

Pseudocumene 338 

Purpurin 4^5,  468 

Pyridine,  297,  328,  471,  472  ;  alkaloids 
derived  from,  488 ;  constitution  of, 
473 ;  derivatives,  isomerism  of,  475  ; 
homologues  of,  478 ;  hydrochloride, 
473 ;  methiodide,  473 ;  platino- 


INDEX. 


PAGE 

chloride,  473;  sulphate,  473;  tests 

for 473 

Pyridine-*,3-dicarboxylic  acid 479 

Pyridine-^-carboxylic  acid 49° 

Pyridine-^y-dicarboxylic  acid 483 

Pyridinecarboxylic  acid,  as.,  /3,  j/ 479 

Pyridinecarboxylic  acids 478 

Pyridinemonocarboxylic  acids 479 

Pyrocatechin 398 

Pyrogallic  acid,  Pyrogallol 400 

Pyrogallolcarboxylic  acid 439 

Pyrogalloldimethyl  ether 400 

Quinic  acid 492 

Quinine;  dimethiodide ;  sulphate 492 

Quinine,  tests  for 493 

Quininic  acid 492 

Quinol 399 

Quinoline,  328,  471,  480;  alkaloids 
derived  from,  492 ;  bichromate,  481 ; 
y-carboxylic  acid,  493 ;  constitu- 
tion of,  481 ;  hydrochloride,  481  ; 
methiodide,  481  ;  platinochloride, 

481  ;  sulphate 4Sl 

Quinolinic  acid,  479,  482  ;  anhydride48o 

Quinone,  413  ;  constitution  of 414 

Quinone  chlorimides 416 

Quinone  dichlorodiimides 416 

Quinonedioxime 414 

Quinonemonoxime 414 

Quinones 413 

Racemic  acid 539,  541 

Racemic  modification 536 

Racemic  .modifications,  resolution 

of. 54i,  543 

Reimer's  reaction 409,  435 

Resorcin  yellow 525 

Resorcinol 398 

Resorcylic  acids,  the 434 

Rocellin 525 

Rosaniline,  511,  513;  base  of,  511; 

chloride,  511  ;  constitution  of 511 

Rosolic  acid 518 

Rubery thric  acid 465 

Saccharin 423 

Salicin 404 

Salicyl  alcohol 404 

Salicylaldehyde 409 

Salicylic  acid,  437  ;  salts  of. 438 

Saligenin,  409,  404;  methyl  ether. ...404 


PAGE 
Sandmeyer's  reaction— 

347,  348,  372,  421,  423 

Sarcolactic  acid 533.  534 

Scarlet  R 525 

Secondary  aromatic  bases 483 

Side-chains 326 

Silver  theobromine 498 

Skraup's  reaction 480,  500 

Sodium  ammonium  racemate 540 

Sodium  dinitro-«-naphthol 527 

Sodium  phenate 390 

Sodium  phenylcarbonate 434,  437 

Sodium  picrate 394 

Stereo-chemical  isomerides 535 

Stereo-isomerism 

Stilbene  ;  dibromide 7^7x^469 

Storax 403. 

Strychnine ;    test    for,    494 ;    hydro- 
chloride,  494  ;  methiodide 494 

Styrolene 432 

Substitution,  rule  of 352 

Sulphanilic  acid 383 

Sulphobenzoic  acid,  /;/,  o,  p 422 

Sulphonamides 38 1 

Sulphonation 380 

Sulphonic  acids,  325,  379;  chlorides.  381 

Tannic  acid 44° 

Tannin 440,  488,  506 

Tartaric  acids,  stereo-isomerism  of. .  .539 

Taurine 5°* 

Terephthalic  acid 339,  427 

Tertiary  aromatic  bases 483 

Tetrabromethane 461 

TetrabromofluoresceTn 521 

Tetrachlorohydroquinone 416 

Tetrachloroquinone 416 

Tetrahydrobenzene 309,  326 

Tetrahydro-/3-naphthylamine 450 

Tetrahydrohydroxyquinoline 499 

Tetramethyldiamidotriphenyl  car- 

binol 508,  509,  510 

Tetramethyl-/-diamidotriphenyl- 

methane 509 

Tetrazodiphenyl  chloride 526 

Tetrazoditolyl  salts 526 

Tetriodofluorescei'n 521 

Thalline 499 

Thebai'ne 495 

TheTne 497 

Theobromine 498 

Thiophen 300 


INDEX. 


PAGE 

Thiotolene 334,  471 

Thymol 339,  397 

Tobacco,  alkaloid  of 489 

Tolidine 379,  526 

Toluene,  297,  334;  chlorination  of  ...342 

Toluenesulphonimide,  o 422 

Toluenesulphonic  acid,  o 422 

Toluenesulphonic  acids 383 

Toluic  acid,  429  ;  ;«,  <?,  /,  337,  423 ;  /,  339 

Toluidine  m,  o}  p 357,  364 

Tolunitriles 422 

Toluquinone 415 

Toluyl  chloride,  348  ;  radicle 333 

Toluylenediamine,  / 415 

Triamidoazobenzene 524 

Triamidoazobenzene  hydrochloride  .  .524 

Triamido-compounds 360 

Triamidotolyldiphenyl  carbinol — 

511,  513,  514 
Triamidotolyldiphenyl      carbinol 

chloride 513 

Triamidotolyldiphenylmethane 511 

Triamidotriphenyl  carbinol 511,  512 

Triamidotriphenyl  carbinol  chloride.  .512 
Triamidotriphenylmethane— - 

341,  511,  512,  513 
Triamidotriphenylmethane  hydro- 
chloride 512 

Tribenzylamine 369 

Tribromaniline 363 

Tribromobenzene 324 

Tribromophenol 392 

Tribromoresorcinol 399 

Trichloraniline 363 

Tridiazotriphenylmethane  chloride  ..512 

Triethylbenzene 324 

Triethylrosaniline  chloride 515 

Trihydric  phenols 399 

a/Sa'-Trihydroxyanthraquinone 468 

Trihydroxyanthraquinones 468 

Trihydroxybenzene,  asymmetrical  ...401 
Trihydroxybenzene,  symmetrical.  400, 401 

viii 


PAGE 

Trihydroxytolyldiphenyl  carbinol 518 

Trihydroxy triphenyl  carbinol 518 

Trimesic  acid 338 

Trimethylbenzene,     adjacent,     338  ; 
asymmetrical,  338;  symmetrical.  ..337 

Trimethylene  bromide 477 

Trimethylene  cyanide 477 

Trimethylpyridines 478 

Trimethylrosaniline  chloride 515 

Trinitrobenzene,  symmetrical ...  354,  395 

Trinitromesitylene 338 

Trinitrophenol 3^4 

Trinitrotriphenylmethane 341,  513 

Triphenyl  carbinol 341 

Triphenylamine 359,  368 

Triphenylcarbinol-0-carboxylic  acid.. 519 

Triphenylmethane 340,  519 

Triphenylmethane,  derivatives  of. ...508 
Triphenylmethane-0-carboxylic  acid-sig 

Triphenylrosaniline  chloride 517 

Tropaeolin  O 525 

Tropic  acid 491 

Tropine 491 

Uranin 521 

Uric  acid 498 

Uvitic  acid 338 

Veratrol 398 

Victoria  green 510 

Vinyltrimethylammonium  hydroxide. 501 

Water  blue 518 

Xylene,   297  ;   bromination  of,   342  ; 

m,o,p 335,  336 

Xylyl  bromide,  m,  423 ;  diethyl 

ether,  m,  426  ;  ethyl  ether,  m,  423 ; 

radicle 334 

Xy lylene  radicle 334 

Zeisel's  method 486,  492 


THE    END. 


Edinburgh : 
Printed  by  W.  &  R.  Chambers,  Limited. 


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